provide external source control URL for schema publish

[n1ru4l]

We already support linking github metadata with a schema publish via the --github flag.

However, not everyone is using GitHub. For these users we can provide a different flag for associating a link with a subgraph publish.

hive schema:publish \
  --commit c7cd95c28de78a92de2250ed34c2aa8fb987ffc3 \
  --external-url https://codeberg.org/foo/bars/commit/c7cd95c28de78a92de2250ed34c2aa8fb987ffc3 \
# ... other parameters

We can then leverage that link in the UI to give users a quick way to jump to their source control provider.

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Consideration:

• The URL path bath must be allow-listed in the target/project/orgs settings by an admin; to avoid sus links being displayed to users