[Bug]: REST `/tasks/{id}:subscribe` uses GET method instead of POST

[ishymko]

It should be POST as per the spec: 0.3, 1.0.

Applies to both 0.3 version from main and 1.0 RC from 1.0-dev, to both client and server.

BUT fixing it for 0.3 is a bit dangerous as one may had assumptions around wrong implementation, so the proposal is:

1. For 1.0 server and client it should just be fixed.
2. 1.0 client in compat 0.3 mode should use POST method as per the spec to be compatible with other SDKs which implement it properly. We may have a flag in the legacy compat transport to use GET if one needs to talk to old Python server which uses wrong method.
3. 0.3 server should accept both to work with old clients.