Get formatted content of a wiki page

Wiki pages usually contain formatted text. If this text is formatted with Markdown or Asciidoc, then GitLab uses its own rendering system to convert the raw text to HTML. However, there is a fallback mechanism for other (which??) formatting systems where we let Gollum generate the HTML.

Currently our Wiki RPC's (which wrap Gollum) do not expose this 'formatted content' field. I think we should add a new RPC that lets the caller retrieve the 'formatted content' for a wiki page.

More information in https://gitlab.com/gitlab-org/gitlab-ce/issues/39805.

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Stages:

• RPC Design: #957 (closed)

• ~"Client Implementation": #958 (closed)

• Server Implementation: #959 (closed)

• ~"Acceptance Testing": #1010 (closed)

RPC Endpoints:

• Endpoint::Name

Known Client Routes:

• Known client endpoints

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