pw_console: Create module and entry point

This CL adds the `pw console` plugin command used to invoke
pw console on the command line and the embed() fuction. At
this point it does nothing. Follow up CLs will build up the
user interface and additional functionality.

Change-Id: I564541cf35c87634ece40604df1afe965f4bf601
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/48642
Pigweed-Auto-Submit: Anthony DiGirolamo <tonymd@google.com>
Reviewed-by: Joe Ethier <jethier@google.com>
Reviewed-by: Keir Mierle <keir@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/PW_PLUGINS b/PW_PLUGINS
index b7e8fe4..a3047ae 100644
--- a/PW_PLUGINS
+++ b/PW_PLUGINS
@@ -16,3 +16,4 @@
 presubmit pw_presubmit.pigweed_presubmit main
 requires pw_cli.requires main
 rpc pw_hdlc.rpc_console main
+console pw_console.__main__ main