Add erase repository content mutation

[rohitvinnakota-codecov]

Adds a mutation to erase a repository's content, replacing the REST endpoint we currently have in place.

A future PR will update gazebo to call this GQL mutation, and a follow up to that will remove the old REST code once we confirm it is no longer in use.

This PR closes https://github.com/orgs/codecov/projects/31/views/11?pane=issue&itemId=63021740.

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