Log the timestamp of accepting homeserver terms of service

This issue has been migrated from [#10769](https://github.com/matrix-org/synapse/issues/10769).

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Synapse can already require a user to accept the terms of service before they are able to message other users, and it is possible to check that this has been done, and what version has been accepted. However, from an audit-trail perspective, we do not have the "when" to go with the "what".

I think this could come up under certain circumstances - e.g. a user is warned over ToS violations, and they dispute the signing of the ToS. Being able to say "yes, you did, at X time" makes for a very clear statement. It is possible to construct an estimate of this from when they started joining other rooms, but an official log would make this process much smoother.