`/createRoom`'s `creation_content` should be validated against the event schema, and rejected if invalid

[turt2live]

Description

When creating a room via /createRoom, creation_content can be specified to append information to the m.room.create event in the room. For known fields like predecessor, it can be important that the correct value type is used, otherwise there may be issues downstream.

Synapse should validate the fields it knows about, and continue to accept unknown fields verbatim.

Related: #13664

Steps to reproduce

• Create a room with predecessor: "some string" instead of predecessor: { object required by spec }

Homeserver

matrix.org

Synapse Version

matrix.org

Installation Method

Other (please mention below)

Database

postgresql

Workers

Multiple workers

Platform

matrix.org

Configuration

No response

Relevant log output

matrix.org

Anything else that would be useful to know?

No response