[master] Add proxy option to gitfs and git_pillar

[eisenmann-b1]

What does this PR do?

This PR add the options gitfs_proxy, git_pillar_proxy and winrepo_proxy, with which a proxy server can be specified to be used for connecting to git repositories. These options can also be set per remote as proxy, overriding the global options.

For Pygit2, a warning will be given if proxy is set and the URL to the git repository does not have an https scheme.
This is due to an libgit2/libgit2#5650 in libgit2, the library pygit2 is using.

What issues does this PR fix or reference?

Fixes #30990

Previous Behavior

There is no option to specify the proxy server. The workaround described in #30990 requires changes to code akin to the ones here, or setting environment variables in service files.

New Behavior

A proxy server can now be specified like this:

gitfs_remotes:
  - https://foo.com/bar.git:
    - proxy: http://proxy.com:8080/

In this example, the repository at https://foo.com/bar.git is fetched through the proxy http://proxy.com:8080/.

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• Docs
• Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• Tests written/updated

Commits signed with GPG?

Yes

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[dmurphy18]

@eisenmann-b1 Need to tick the Tests written/updated box too

[eisenmann-b1]

@eisenmann-b1 Need to tick the Tests written/updated box too

I did not tick it since there are no new tests, which I felt would be necessary for this.

[dmurphy18]

@eisenmann-b1 you updated tests, hence you need to tick the box, for example: tests/pytests/unit/fileserver/gitfs/test_gitfs.py

[eisenmann-b1]

@dmurphy18 ok, I ticked the box.

[eisenmann-b1]

Added the option to winrepo.