Add header to flag API POST requests with no write intentions

Add header to flag API POST requests with no write intentions

This performs sanity check that request *is* for a non-write module.
By handling the validation, the CDN layer can simply use the presence
of this header to route POST requests to the local datacenter.
Without validation, users could cause strange traffic patterns and slow
cross-DC database writes (which can involve many RTTs).

This is useful for AJAX widgets that need to post a payload to get a
response, but that don't actually change anything in the process. They
should be able to use the local datacenter.

Bug: T91820
Change-Id: I34248ddee33033e3d1d86c3391236259d917d4a7