Add support for SAML SSO authorization APIs

[joaopenteado]

This PR adds support for both ListCredentialAuthorizations and RemoveCredentialAuthorization methods available for the GitHub Enterprise Cloud REST API for SAML SSO authorizations.

Listing and deleting credential authorizations is available to organizations with GitHub Enterprise Cloud.

[gmlewis]

Thank you, @joaopenteado !
Just a few tweaks, please, then we will be ready for a second LGTM+Approval from any other contributor to this repo before merging.

[codecov]

Codecov Report

Merging #2835 (529eb79) into master (b02bb75) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@ Coverage Diff @@## master #2835 +/- ##
=======================================
Coverage 98.06% 98.06% =======================================
Files 137 138 +1 Lines 12291 12314 +23 =======================================
+ Hits 12053 12076 +23 
Misses 162 162 Partials 76 76 

Impacted Files	Coverage Δ
github/orgs_credential_authorizations.go	100.00% <100.00%> (ø)

[gmlewis]

Thank you, @joaopenteado !
LGTM.

Awaiting second LGTM+Approval from any other contributor to this repo before merging.

[joaopenteado]

Thank you for the fast review and comments as always, @gmlewis!
I just committed a improvement to the test for ListCredentialAuthorizations in order to match what was being done in other functions.

[gmlewis]

Awesome, @joaopenteado ! Thank you very much!
I was wondering why the coverage dropped, but it looks like you found it and fixed it!
🎉

[o-sama]

LGTM too :)

[gmlewis]

Thank you, @o-sama !
Merging.