pw_rpc: add EchoService

This defines and implements a basic echo service to facilitate testing
a pw_rpc integration on a device.

Change-Id: I18ff90389aa0cdce238717f4c2cc7c59b7a3f32e
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/14060
Commit-Queue: Alexei Frolov <frolv@google.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>
Reviewed-by: Keir Mierle <keir@google.com>
diff --git a/pw_rpc/BUILD.gn b/pw_rpc/BUILD.gn
index 14aa95c..90fc36e 100644
--- a/pw_rpc/BUILD.gn
+++ b/pw_rpc/BUILD.gn
@@ -140,6 +140,11 @@
   sources = [ "pw_rpc_protos/packet.proto" ]
 }
 
+pw_proto_library("echo_service_proto") {
+  sources = [ "pw_rpc_protos/echo.proto" ]
+  inputs = [ "pw_rpc_protos/echo.options" ]
+}
+
 # Source files for pw_protobuf's protoc plugin.
 pw_input_group("nanopb_protoc_plugin") {
   inputs = [
@@ -152,7 +157,11 @@
 
 pw_doc_group("docs") {
   sources = [ "docs.rst" ]
-  inputs = [ "pw_rpc_protos/packet.proto" ]
+  inputs = [
+    "pw_rpc_protos/echo.proto",
+    "pw_rpc_protos/packet.proto",
+  ]
+  group_deps = [ "nanopb:docs" ]
 }
 
 pw_test_group("tests") {