Move space summaries out of "MSC land"

[matrixbot]

This issue was originally created by @S7evinK at matrix-org/dendrite#3096.

While we already have MSC2946, it is still needed to manually enable space summaries.
We should move it to the fitting spots (e.g. clientapi/federationapi/roomserver), given it is in the spec since version 1.2.
It's also a source of confusion for new users, having to enable the MSC manually.

[matrixbot]

This comment was originally posted by @swedgwood at matrix-org/dendrite#3096 (comment).

Space summaries are no longer hidden behind an MSC flag :) (matrix-org/dendrite#3134)