plugins, runtime: Add visibility for server init

- Plugins can now access a channel which receives a message when the OPA
  server is fully initialized and ready to receive traffic.
- Added ServerInitialized() and ServerInitializedChannel() to plugin
  manager.
- Runtime now calls ServerInitialized() when server listeners are
  initialized.

Fixes #3701

Signed-off-by: Grant Shively <gshively@godaddy.com>