Add ProjectCard attribute to events model

[maxim-lobanov]

Hello!
I have faced with issue that Event model doesn't contain project_card property that is important for all events that are related to GitHub boards (Projects).
For example, for event added_to_project, it contains the name of column where issue is added.
For event moved_columns_in_project, it contains the previous and new column names.
This PR includes the following changes:

1. Change EventInfo model to IssueEvent (please see note below for details)
2. Add model IssueEventProjectCard accordingly to GitHub API description
3. Add header IssueEventsApiPreview = "application/vnd.github.starfox-preview" that is needed to get project_card value
4. Update tests
5. Update Octokit.Reactive with the same changes

Note: Why EventInfo model was replaced by IssueEvent in IssueClient:
These models are pretty similar but IssueEvent contains additional fields like Issue and ProjectCard.
I am not sure why some methods in IssueClient used IssueEvent but some methods used EventInfo. Based on API, all methods in IssueClient should use IssueEvent.

[codecov]

Codecov Report

Merging #2102 into master will decrease coverage by 0.05%.
The diff coverage is 57.14%.

@@            Coverage Diff            @@
##           master   #2102      +/-   ##
=========================================
- Coverage   67.05%     67%   -0.06%     
=========================================
  Files         538     539       +1     
  Lines       14141   14185      +44     
=========================================
+ Hits         9482    9504      +22     
- Misses       4659    4681      +22

Impacted Files	Coverage Δ
Octokit/Models/Response/EventInfo.cs	4.54% <ø> (ø)	⬆️
Octokit/Helpers/AcceptHeaders.cs	100% <ø> (ø)	⬆️
Octokit/Models/Response/TimelineEventInfo.cs	3.33% <0%> (-0.24%)	⬇️
Octokit/Models/Response/IssueEvent.cs	6.66% <0%> (-1.03%)	⬇️
Octokit/Models/Response/IssueEventProjectCard.cs	0% <0%> (ø)
...t.Reactive/Clients/ObservableIssuesEventsClient.cs	100% <100%> (ø)	⬆️
Octokit/Clients/IssueTimelineClient.cs	100% <100%> (ø)	⬆️
Octokit/Clients/IssuesEventsClient.cs	100% <100%> (ø)	⬆️

[maxim-lobanov]

@shiftkey , could you please take a look at this PR?
I have noticed that some test coverage checks failed due to new model that I have introduced for the project cards. Since it is just API models I am not sure what tests we need here so we should be okay with these changes.
If there is anything that should be improved, please let me know

[shiftkey]

I have noticed that some test coverage checks failed due to new model that I have introduced for the project cards. Since it is just API models I am not sure what tests we need here so we should be okay with these changes.

This is fine. I think I need to tweak the Codecov settings to focus on certain namespaces over others.

I am not sure why some methods in IssueClient used IssueEvent but some methods used EventInfo. Based on API, all methods in IssueClient should use IssueEvent.

I am not sure about this either, and need to check the history to understand why these are different.

[maxim-lobanov]

I am not sure about this either, and need to check the history to understand why these are different.

@shiftkey ,
Unfortunately, nothing is in history. It was introduced with the first commit when IssueEventClient was implemented.

I believe that my change is safe enough because:

1. EventInfo model:

public long Id { get; protected set; }
public string NodeId { get; protected set; }
public string Url { get; protected set; }
public User Actor { get; protected set; }
public User Assignee { get; protected set; }
public Label Label { get; protected set; }
public StringEnum<EventInfoState> Event { get; protected set; }
public string CommitId { get; protected set; }
public DateTimeOffset CreatedAt { get; protected set; }

2. IssueEvent model:

public long Id { get; protected set; }
public string NodeId { get; protected set; }
public string Url { get; protected set; }
public User Actor { get; protected set; }
public User Assignee { get; protected set; }
public Label Label { get; protected set; }
public StringEnum<EventInfoState> Event { get; protected set; }
public string CommitId { get; protected set; }
public string CommitUrl { get; protected set; }
public DateTimeOffset CreatedAt { get; protected set; }
public Issue Issue { get; protected set; }

As you can see, IssueEvent contains all fields that EventInfo has.
But, also, it contains the following additional fields (which exist only for events that come with issues) :

1. public string CommitUrl { get; protected set; }
2. public Issue Issue { get; protected set; }
   So we just extend model.

I have changed EventInfo-> IssueEvent for the method GetAllForIssue. Based on GitHub API, this method should return Issue field so we should use IssueEvent model here.

[shiftkey]

@maxim-lobanov this is looking really great - thanks for the contribution.

Only bit of feedback I have is about updating the tests to assert on the preview accept header, everything else looks great!

[shiftkey]

release_notes: added ProjectCard property to IssueEvent response model