Opt-in for non-GET endpoints

[Dreamsorcerer]

We should consider making #452 an opt-in feature in v3.
I've seen backend developers create a POST endpoint for SSE, and then frontend developers point out that they can't use it.
So, it might make sense to disallow non-GET endpoints by default again, so developers don't make this mistake.

[Olegt0rr]

Frontend can't use non-GET methods with default EventSource. But with other sse-clients they can use other methods.

Original request: #414 (comment)

[Olegt0rr]

Do you wanna implement it as a feature-toggle?
allow_all_methods: bool

[Dreamsorcerer]

Yeah, probably something like that.

[Olegt0rr]

What about something like this? #543