fix: add POST support in frontend API as well

[ivarconr]

Describe the feature request

The Frontend API does not support POST request (as an alternative to GET). This is supported by Unleash Edge and can cause confusion for users.

https://github.com/Unleash/unleash/blob/main/src/lib/features/frontend-api/frontend-api-controller.ts#L83-L88

Background

No response

Solution suggestions

No response

[ivarconr]

Some more context:
The default way to use the frontend API is to perform a HTTP "GET" request where the unleash context is passed as query parmeters. E.g.

/api/frontend?userId=123&region=us

The fronted SDKs also support sending a HTTP "POST" request instead, where the context is added as part of the body. We already have an implementation of this in the proxy. The context will then be part of the http body and follow this schema:

example body:

"context": {
  userId: "123"
}

Also note that the client can request only a subset of the flag it cares about through the toggles property, which is a list of feature flag names.