Expose message retention to clients

Synapse has a configuration flag to control if retention is enabled. This should be exposed to clients via the `/configuration` endpoint of [MSC1763](https://github.com/matrix-org/matrix-spec-proposals/pull/1763).

More context can be found in: https://github.com/element-hq/element-enterprise/issues/312

Start by locating Synapse's implementation of the /configuration endpoint and the configuration flag controlling message retention, then compare its response with MSC1763. Expose the retention setting through that endpoint and add or update coverage so the response reflects whether retention is enabled.

Written by the indexing model from the issue text.