Add `--git-metadata` flag to `buf push`

[doriable]

This PR adds the --git-metadata flag to buf push:

• this flag is a "meta-flag" that uses Git source control state to help set --label flag(s), --source-control-url, and --create-default-label to the HEAD branch of the default Git remote
• this flag is only compatible with checkouts of Git source repositories
• this flag does not allow you to set --source-control-url, --create-default-label, --label, --tag, --branch, or --draft labels alongside

This PR also changes the default visibility for --create-visibility to "private". This means that users are no longer required to specify the --create-visibility flag when calling buf push --create -- it will default to creating a private repository if one does not already exist.

Also, this PR includes a changelog entry for all the changes above, as well as the --label, --create-default-label, and --source-control-url flags.