Add entropy facade in pw_tls_client for users to provide an entropy
source.
Change-Id: I0576971ea0f69f8967658db3270bcbdd3dd89668
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/50161
Commit-Queue: Yecheng Zhao <zyecheng@google.com>
Reviewed-by: Ali Zhang <alizhang@google.com>