pw_tls_client: Add a build time backend

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>
diff --git a/pw_tls_client/BUILD b/pw_tls_client/BUILD
index d23ebc6..5b06962 100644
--- a/pw_tls_client/BUILD
+++ b/pw_tls_client/BUILD
@@ -58,3 +58,14 @@
         "//pw_status",
     ],
 )
+
+# TODO(zyecheng): The target requires a build_time.h header that defines a
+# 'constexpr size_t kBuildTimeMicrosecondsUTC' variable for storing the build time.
+# In gn build, this is generated by a python action target. Need to figure out a
+# solution in bazel build.
+pw_cc_library(
+    name = "build_time",
+    srcs = [
+        "build_time.cc",
+    ],
+)