client app: added common udp_wait_readready function

[maxsharabayko]

There is a duplicated code in the client application, which implements waiting for read-readiness on a UDP socket.
This code is combined into a single function to reduce code duplication and cognitive complexity.

udp_wait_readready(int fd, quicly_conn_t** connections, size_t num_connections, int64_t client_enqueue_rq_at)

[maxsharabayko]

Here in case of client the connection context was used instead of the global ctx. Does it make any difference?

quicly_context_t *ctx = quicly_get_context(conn);