Migrate Repository#license_key to Gitaly

Original issue description:

You wouldn't know it by the innocent name, but the licensee gem actually depends on Rugged as well. See https://gitlab.com/gitlab-org/gitlab-ce/issues/43268#note_59093108 for more details.

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Stages:

• RPC Design: gitaly-proto!169 (merged)

• Server Implementation: !601 (merged)

• ~"Client Implementation": gitlab-org/gitlab-ce!17449

• ~"Acceptance Testing": #1058 (closed)

RPC Endpoints:

• Repository#FindLicense

Known Client Routes:

• Repository#license_key