Allow opting out of consent when using shared secret registration

[Half-Shot]

Ideally via a flag in the body of the request.

Currently when bots try to register themselves on a homeserver with consent enabled, they cannot them go ahead and sync because consent is required to continue. In my case, the bots are running in an environment where human interaction isn't possible and the bots need to be able to be created without intervention.

[Half-Shot]

Believe #5902 fixed this.