When we first integrated mbedTLS, we decided not to enable
MBEDTLS_THREADING_C (which adds mutex locking to calls modifying the
state), and instead to simply create separate contexts ("states") for
each connection.
This worked fine until recently.
Sadly, mbedTLS 3 added a global state for the new PSA crypto
functionalities (which are required to support TLSv1.3).
This results in TLSv1.3 connections to access and modify the global
state concurrently when running in threads.
This commit enables MBEDTLS_THREADING_C, and MBEDTLS_THREADING_C_ALT to
provide a generic Godot implementation using the engine Mutex class.
2c789788c0