MSC3997: Add timestamp massaging to /createRoom

[MadLittleMods]

MSC3997: Add timestamp massaging to /createRoom?ts=123

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Server implementations:

• Implementations needed

Other related references:

• Original MSC where timestamp massaging was introduced: MSC3316
  • add ?ts massaging for ASes synapse#2754
  • Add query parameter ts to allow appservices set the origin_server_ts for state events synapse#11866
• Synapse issue tracking /createRoom timestamp massaging proposed in this MSC: Add timestamp massaging to /createRoom synapse#15346
• Related MSC3998 to add timestamp massaging to /join and /knock

[MadLittleMods]

cc @ara4n as the original ?ts proposer from the old version of MSC2716

cc @tulir as an interested party in timestamp massaging (author of MSC3316 and one of the main consumers of the ?ts API)