Faster remote room joins: add an error code to `/members` when in partial state

This issue has been migrated from [#14587](https://github.com/matrix-org/synapse/issues/14587).

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For now the request is just stalled until the join completes, which is not ideal since clients are not aware of what is happening.

This requires a MSC to define the error code.
For backward compatibility we also probably need to add a parameter to `/members` so that the client can opt-in to the specific error code.
Or perhaps returning a 5xx is enough ? https://stackoverflow.com/questions/9794696/which-http-status-code-means-not-ready-yet-try-again-later has several suggestions on that subject.

Start by reviewing the `/members` endpoint and the Matrix Spec Change (MSC) process, since the issue says a new error code must be defined. Resolve whether clients need an opt-in parameter or a 5xx response, then document the chosen compatibility behavior and error semantics as done.

Written by the indexing model from the issue text.