Code Quality: Refactored LibGit2 integration as a DI service for future-proofing

[Lamparter]

Resolved / Related Issues

• Related Feature: Use installed git.exe instead of libgit2 #16738

Steps used to test these changes

The implementation is logically identical to the previous, it has just been moved.

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In this PR, I:

• converted the previous LibGit2.cs (that aims to implement IVersionControl) into a DI service named LibGit2Service
• fixed the IsExecutingGitAction field as well as both event handlers IsExecutingGitActionChanged and GitFetchCompleted, as raised by @yair100 in Code Quality: Moved Checkout operation into Git abstraction #18669 (comment)
• moved FetchOrigin to the new service (because it invokes the event handler, it must be part of the type containing the event handler as per C# convention)

this future proofs the git abstraction because we will want to move to a DI service later anyway (because the end goal is to allow the user to choose how the app controls git, i.e. they can switch between libgit2sharp and git.exe) and fixes the issue Yair raised before at the same time :)