Add CustomMessages demo to server and agent examples

[juandemanjon]

• CustomMessages are a key extension mechanism in the OpAMP spec, enabling vendor-specific or experimental features without changing the base protocol.
• Today there is no self-contained, runnable example in opamp-go illustrating how to construct, send, and process CustomMessages end-to-end.
• Providing a small demo will:
  • Help new adopters understand the pattern quickly (like me)
  • Encourage consistent, spec-aligned usage and reduce friction when building real-world server or agent apps.