Implement a backend to facade pw_tls_client:time that uses build time
as the date time.
Change-Id: I5e2ed16198005c7e337e0cd8fcdeb08e977338c1
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/51044
Commit-Queue: Yecheng Zhao <zyecheng@google.com>
Reviewed-by: Ali Zhang <alizhang@google.com>
