Change `git.version` metric value type to `object`

What does this MR do and why?

Related to #351898 (closed).

This changes the metric value type of git.version to object, and adds a JSON schema alongside it.

Also, fixes the metric definition specs to have alt_usage_data to return actual fallback values. git.version previously returned just -1 instead of { major: -1 }, which tripped up the JSON schema and failed a spec.

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