client: Get, set delete and parse pushers

This follows

https://spec.matrix.org/legacy/push_gateway/r0.1.1#post-matrix-push-v1-notify https://spec.matrix.org/v1.11/client-server-api/#post_matrixclientv3pushersset

and allows us get and set a clients currently configured pushers.

Only missing bit (which is now rather quick) is deleting pushers.

(we use /r0/ for the API version like other URLs in libcmatrix atm)