Custom git registry to work with branches

[srgbtl]

---

Is your feature request related to a problem? Please describe.
As a developer, I want to be sure that a new port in the custom git registry is consumable before merging to main.

1. New branch is created in custom registry repo and the new port is added
2. In repo that uses this registry I am changing baseline to point a commit in branch and build the project
3. If the project is successfully built I am sure I can merge my changes to the main branch.

Proposed solution
Add a new field to Registry object to specify a branch to use. If the field is not specified HEAD is used.

Describe alternatives you've considered
N/A

Additional context
N/A

[srgbtl]

Just found #17726 and microsoft/vcpkg-tool#99
@strega-nil-ms is there any progress on this PR? I might help with this PR. I have an environment ready to test this.
CC: @BillyONeal @ras0219-msft

[strega-nil-ms]

@srgbtl unfortunately, I've been busy with other stuff right now, so I've not been able to work on it; if you'd be up for it, it'd be great if you could take up the torch.

[srgbtl]

@BillyONeal @ras0219-msft Looking forward to hear any feedback and/or vision on this issue. We really need this feature, and I'm wondering whether to switch to fork to use it.

[srgbtl]

New key was added in microsoft/vcpkg-tool#155
Usage:

    {
        "default-registry": null,
        "registries": [
            {
                "kind": "git",
                "repository": "myrepourl",
                "baseline": "baseline1",
                "reference": "ref1",
                "packages": ["vcpkg-internal-e2e-test-port"]
            },
            {
                "kind": "git",
                "repository": "myrepourl",
                "baseline": "baseline2",
                "reference": "ref2",
                "packages": ["vcpkg-internal-e2e-test-port2"]
            }
        ]
    }