feat(events): add events management capabilities

This change introduces some new plugins which provide the basics
for managing events inside of backstage.
Hereby, it offers extension points to add event publishers and subscribers
as well as to exchange the event broker implementation.

- `@backstage/plugin-events-backend`:
  backend for the events management which connects all parts
  and provides a simple in-memory event broker
- `@backstage/plugin-events-node`:
  interfaces and API for `@backstage/plugin-events-backend`
- `@backstage/plugin-events-test-utils`:
  test utilities like implementations useful for writing tests at modules

All plugins support the new backend-plugin-api.

Relates-to: #11082
Signed-off-by: Patrick Jungermann <Patrick.Jungermann@gmail.com>

plugins/events-node/README.md

@@ -0,0 +1,3 @@
+# plugin-events-node
+
+Houses types and utilities for building events-related modules.