mark principal and issuer class under pkg/identity as deprecated

[heysujal]

Summary

Closes #1798

Release Note

Added Deprecation warning for Principal and Issuer class under pkg/identity as suggested here

#1798 (comment)

Documentation

I think it requires adding a documentation.

[bobcallaway]

LGTM, I'd like @haydentherapper to take a look when he's back.

[Hayden-IO]

Thanks for working on this. These interfaces are not deprecated though. The comment in the linked issue was for deprecating the issuer and principal structs under https://github.com/sigstore/fulcio/tree/main/pkg/identity/codefresh, https://github.com/sigstore/fulcio/tree/main/pkg/identity/buildkite, https://github.com/sigstore/fulcio/tree/main/pkg/identity/gitlabcom, and https://github.com/sigstore/fulcio/tree/main/pkg/identity/github.

I would mark any publicly exported method/struct as deprecated and link to the respective method/struct under https://github.com/sigstore/fulcio/blob/main/pkg/identity/ciprovider/.

[heysujal]

I have a query that all the structs under the mentioned directories like github and gitlab etc are non-public. Then ideally I shouldn't have marked them as deprecated but then what structs need to marked as deprecated.

[codecov]

Codecov Report

Attention: Patch coverage is 57.14286% with 3 lines in your changes missing coverage. Please review.

Project coverage is 43.62%. Comparing base (cf238ac) to head (6b13035).
Report is 326 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/challenges/challenges.go	0.00%	3 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@ ## main #1980 +/- ## =========================================== - Coverage 57.93% 43.62% -14.32%  =========================================== Files 50 73 +23 Lines 3119 5724 +2605 =========================================== + Hits 1807 2497 +690  - Misses 1154 2997 +1843  - Partials 158 230 +72 

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[Hayden-IO]

You'll need to add some comments to have the linter ignore where we still use the deprecated methods.