ETag support for /_matrix/client/v3/rooms/{roomId}/state

For https://spec.matrix.org/latest/client-server-api/#get_matrixclientv3roomsroomidstate, https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag

Similar to #2019 , it would be nice to know if the state has changed since the last request was made to /state (again for the purposes of integrations moreso than clients). The server could presumably keep a simple incrementing hash value for the current state of a room, and whenever that changes the hash is recalculated. This could be quickly used as an ETag and result in a safer way to cache state for integrations.

Start with the Matrix client-server API section for GET /_matrix/client/v3/rooms/{roomId}/state and compare the related proposal in #2019. Define the ETag and state-change behavior for this endpoint, including how integrations can use it for caching; the work is done when the specification clearly documents the resulting protocol behavior.

Written by the indexing model from the issue text.