Method to run `hg recover`

This Mercurial specific gRPC call would perform the equivalent of hg recover.

We can add it to RepositoryService or put it in a service dedicated to Mercurial, same as in #60 (closed)

• define the protocol
• implement it

For the record here's what the Rails app currently does:

      def hg_recover
        hg_env = hg_env_minimal
        hg_env.update({ 'HGUSER' => 'Heptapod system',
                        'HEPTAPOD_SKIP_ALL_GITLAB_HOOKS' => 'yes' })
        hg_call(%w[recover], env: hg_env, success_code_can_be_one: true)
      end