oauth: Limit JSON parsing depth in the client

oauth: Limit JSON parsing depth in the client

Check the ctx->nested level as we go, to prevent a server from running
the client out of stack space.

The limit we choose when communicating with authorization servers can't
be overly strict, since those servers will continue to add extensions in
their JSON documents which we need to correctly ignore. For the SASL
communication, we can be more conservative, since there are no defined
extensions (and the peer is probably more Postgres code).

Reviewed-by: Aleksander Alekseev <aleksander@timescale.com>
Discussion: https://postgr.es/m/CAOYmi%2Bm71aRUEi0oQE9ciBnBS8xVtMn3CifaPu2kmJzUfhOZgA%40mail.gmail.com