Add RemoteService#RemoteRepositoryExists RPC

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Stages:

• RPC Design: gitaly-proto!177 (merged)

• Server Implementation: !636 (merged)

• ~"Client Implementation": https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18017

RPC Endpoints:

• RemoteService#RemoteReposityExists

Known Client Routes:

• Importing; we'd like to know if we can actually clone

Needed for https://gitlab.com/gitlab-org/gitlab-ce/issues/43929

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