Implement endorsements API

[PattaFeuFeu]

Description

This PR implements the current state of the Endorsements API so that consumers of this library can get Accountss that the user is currently featuring on their profile.

I have changed MockClient to be a wee bit more versatile and malleable when called from tests.

Closes #261.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
  https://github.com/andregasser/bigbone/wiki/Mastodon-API-Coverage#methods needs to be updated after this has been merged!
• Dependency update / replacement

How Has This Been Tested?

gradle check

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have made corresponding changes to the documentation if necessary
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[codecov]

Codecov Report

Merging #286 (8517b96) into master (5dcaea0) will increase coverage by 0.59%.
The diff coverage is 95.45%.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #286      +/-   ##
============================================
+ Coverage     35.74%   36.33%   +0.59%     
- Complexity      348      362      +14     
============================================
  Files           101      103       +2     
  Lines          3047     3069      +22     
  Branches        184      184              
============================================
+ Hits           1089     1115      +26     
+ Misses         1838     1833       -5     
- Partials        120      121       +1     

Files	Coverage Δ
...n/kotlin/social/bigbone/rx/RxEndorsementMethods.kt	100.00% <100.00%> (ø)
...e/src/main/kotlin/social/bigbone/MastodonClient.kt	32.01% <100.00%> (+0.24%)	⬆️
...in/social/bigbone/api/method/EndorsementMethods.kt	90.90% <90.90%> (ø)

... and 1 file with indirect coverage changes

[andregasser]

Looks good to me!

[andregasser]

👍