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 https://github.com/hub4j/github-api/issues/1828

Before submitting a PR:

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

When creating a PR:

• [x] Fill in the "Description" above with clear summary of the changes. This includes:
  • [x] If this PR fixes one or more issues, include "Fixes #" lines for each issue.
  • [x] Provide links to relevant documentation on https://docs.github.com/en/rest where possible. If not including links, explain why not.
• [x] 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."
• [x] Enable "Allow edits from maintainers".

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 81.03%. Comparing base (c5ce1f6) to head (c2ac4b0). Report is 9 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1835 +/- ## ============================================ + Coverage 80.64% 81.03% +0.38% - Complexity 2358 2409 +51 ============================================ Files 225 233 +8 Lines 7203 7344 +141 Branches 395 399 +4 ============================================ + Hits 5809 5951 +142 + Misses 1149 1148 -1 Partials 245 245 ```

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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)

[mestebangutierrez]

@bitwiseman, is anything else required on my side before continuing the contribution process?

[jbenaventem]

@bitwiseman Please take a moment to review this Pull Request. It contains important changes that could significantly improve the GitHub Enterprise accounts use. If you notice any issues or have any concerns, do not hesitate to comment. Your feedback is greatly appreciated.

[mestebangutierrez]

Very well tested and implemented. Great job!

A few adjustments and renames needed.

@bitwiseman, first of all, thanks for the review!

I've done all the requested changes, addressing each of them in its own commit.

I've also answered to all the conversations, but haven't resolved them. I let that to you in case you consider that that changes I've done satisfy the request.

Again, thanks for your effort and patience!

[mestebangutierrez]

Rebased from main again.