Add filter state object bool proxy_receive_before_connect that tcp_proxy
filter checks for and when set to 'true' will not set readDisable on the
downstream connection before upstream connection is established.

This change allows advanced cases where a network read filter needs to
receive (and possibly send) downstream data that may change the
destination of the upstream TCP connection, e.g. via metadata set based
on the received data. If this is the case, then the read filter preceding
the tcp_proxy filter must return StopIteration from its onNewConnection()
call, causing tcp_proxy filter to postpone its upstream connection
establishment until onData() returns Continue.

Any data reaching the tcp_proxy filter before the upstream connection is
established is buffered so that the downstream filters do not see the
same data again which would be the case if it would remaining in the buffer
and more data is received. This also allows downstream filters inject
data before the upstream connection is established; such injected data
would be lost if tcp_proxy would not buffer it while connection
establishment is still ongoing.

An existing dynamic metadata integration test is modified to use
proxy_receive_before_connect=true. This makes the use case less
reliant on the tcp_proxy internal detail, such as balancing and timing of
the readDisable() calls on the downstream connection.

Signed-off-by: Jarno Rajahalme <jarno@isovalent.com>

Signed-off-by: Akshita Agarwal <akshita.agarwal@airbnb.com>