Provide support for External Group operations

[mestebangutierrez]

Description

Provide support for External Group operations, in particular:

• List external groups in an organization
• Get an external group
• List a connection between an external group and a team
• Connect an external group to a team
• Remove connection between an external group and a team

Fixes #1828

Before submitting a PR:

• Changes must not break binary backwards compatibility. If you are unclear on how to make the change you think is needed while maintaining backward compatibility, CONTRIBUTING.md for details.
• Add JavaDocs and other comments explaining the behavior.
• When adding or updating methods that fetch entities, add @link JavaDoc entries to the relevant documentation on https://docs.github.com/en/rest .
• Add tests that cover any added or changed code. This generally requires capturing snapshot test data. See CONTRIBUTING.md for details.
• Run mvn -D enable-ci clean install site locally. If this command doesn't succeed, your change will not pass CI.
• Push your changes to a branch other than main. You will create your PR from that branch.

When creating a PR:

• Fill in the "Description" above with clear summary of the changes. This includes:
  • If this PR fixes one or more issues, include "Fixes #" lines for each issue.
  • Provide links to relevant documentation on https://docs.github.com/en/rest where possible. If not including links, explain why not.
• All lines of new code should be covered by tests as reported by code coverage. Any lines that are not covered must have PR comments explaining why they cannot be covered. For example, "Reaching this particular exception is hard and is not a particular common scenario."
• Enable "Allow edits from maintainers".

[codecov]

Codecov Report

Attention: Patch coverage is 79.68750% with 26 lines in your changes are missing coverage. Please review.

Project coverage is 80.56%. Comparing base (c5ce1f6) to head (8b4b8c5).
Report is 1 commits behind head on main.

❗ Current head 8b4b8c5 differs from pull request most recent head 8c859c0. Consider uploading reports for the commit 8c859c0 to get more accurate results

Files	Patch %	Lines
.../main/java/org/kohsuke/github/GHExternalGroup.java	65.71%	12 Missing ⚠️
...a/org/kohsuke/github/EnterpriseManagedSupport.java	66.66%	4 Missing and 3 partials ⚠️
...suke/github/GHEnterpriseManagedUsersException.java	50.00%	2 Missing ⚠️
...hub/GHNotExternallyManagedEnterpriseException.java	50.00%	2 Missing ⚠️
...thub/GHTeamCannotBeExternallyManagedException.java	50.00%	2 Missing ⚠️
src/main/java/org/kohsuke/github/GHError.java	66.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1835      +/-   ##
============================================
- Coverage     80.64%   80.56%   -0.08%     
- Complexity     2358     2390      +32     
============================================
  Files           225      232       +7     
  Lines          7203     7318     +115     
  Branches        395      400       +5     
============================================
+ Hits           5809     5896      +87     
- Misses         1149     1173      +24     
- Partials        245      249       +4     

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[mestebangutierrez]

• Rebased from main
• Added additional tests to improve coverage
• Removed unnecessary code (e.g., unrequired bridge methods)
• Minor improvements (make parameters & variables final where possible, define serialVersionUID for serializable classes)