pw_tls_client: Add fake entropy backend

Add a default fake entropy for demo and test purpose.

Change-Id: Icfa4d91cfbdb5590d980656792793755803a0c4c
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/51402
Commit-Queue: Rob Mohr <mohrr@google.com>
Pigweed-Auto-Submit: Rob Mohr <mohrr@google.com>
Reviewed-by: Ali Zhang <alizhang@google.com>
diff --git a/pw_tls_client/BUILD b/pw_tls_client/BUILD
index d688cdc..28d7d3d 100644
--- a/pw_tls_client/BUILD
+++ b/pw_tls_client/BUILD
@@ -48,7 +48,7 @@
 # ready.
 
 pw_cc_facade(
-    name = "pw_tls_client_entropy_facade",
+    name = "entropy_facade",
     hdrs = [
         "public/pw_tls_client/entropy.h",
     ],
@@ -59,6 +59,14 @@
     ],
 )
 
+pw_cc_library(
+    name = "fake_entropy",
+    srcs = [ "fake_entropy.cc" ],
+    deps = [
+        ":entropy_facade"
+    ],
+)
+
 # 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