Support content-type annotation for K8s secrets

[gl-johnson]

Desired Outcome

Add support to K8s secrets mode for setting secret content-types via annotations in the secrets manifest. It should support the following:

• text (default)
• base64

The conjur-map annotation for secrets will now support the format:

conjur-map: |-
  <alias_value>: <var_id_value>               # already supported
  <alias_value>:                              # to be added
    id: <var_id_value>
    content-type: <content_type_value>  

Where id is required to have a non-empty value, and content-type is an optional key which defaults to text unless explicitly set to base64

Implemented Changes

• Parse new YAML annotations into existing updateDestination struct in K8sProvider
• Add tests to check for the expected content-type of secrets via relevant log/error messages

Definition of Done

At least 1 todo must be completed in the sections below for the PR to be
merged.

Changelog

• The CHANGELOG has been updated, or
• This PR does not include user-facing changes and doesn't require a
  CHANGELOG update

Test coverage

• This PR includes new unit and integration tests to go with the code
  changes, or
• The changes in this PR do not require tests

Documentation

• Docs (e.g. READMEs) were updated in this PR
• A follow-up issue to update official docs has been filed here: [insert issue ID]
• This PR does not require updating any documentation

Behavior

• This PR changes product behavior and has been reviewed by a PO, or
• These changes are part of a larger initiative that will be reviewed later, or
• No behavior was changed with this PR

Security

• Security architect has reviewed the changes in this PR,
• These changes are part of a larger initiative with a separate security review, or
• There are no security aspects to these changes

[szh]

Is the actual decoding going to be in a separate PR

[gl-johnson]

I planned on tackling it in a followup task, yes. The decoding implementation should be a minor code change but that PR will also include error handling, additional tests, dev environment/manifest updates, etc. so I thought it would be more manageable as a separate task.

This assertion was copied over from the POC since it'll be helpful for that PR.

[szh]

Needs a rebase, otherwise looks good

[codeclimate]

Code Climate has analyzed commit 062433f and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 92.0% (50% is the threshold).

This pull request will bring the total coverage in the repository to 89.1% (0.0% change).

View more on Code Climate.

[szh]

LGTM!