libpdx_uds: Serialize access to connection socket between threads

Added a mutex to allow only one client thread to perform atomic
send-request/receive-responce actions.

Also added a unit test that perfroms multiple parallel client requests
to the same service to ensure it can handle multithreaded access
correctly.

Bug: 37443070
Test: `libpdx_uds_tests` pass
Change-Id: Ica516f7806f9146fb530b5cb371d2ee89146fed7