Add tested BSR API and webhook examples

[jrinehart-buf]

This PR:

• Lifts the webhooks example and API examples from docs into small BSR examples with tests.
  • Each has CI-focused Makefile, similar to Protovalidate examples, where a default target tests the examples.
• Updates the root Makefile to:
  • include these in CI
  • exclude certain files from the license-header target: we don't want to print the copyright notice at the top of examples in docs, where we'll include these files