RFC: Dynamic webhook middleware for ToolHive (#17)

* RFC: Dynamic webhook middleware for ToolHive

Port proposal from toolhive PR #2300. This RFC introduces a dynamic
webhook middleware system inspired by Kubernetes admission webhooks,
enabling external services to participate in the MCP request pipeline.

Signed-off-by: Juan Antonio Osorio <ozz@stacklok.com>

* Rename RFC file to match PR number (THV-0017)

Signed-off-by: Juan Antonio Osorio <ozz@stacklok.com>

* Address PR feedback on webhook middleware RFC

- Add HMAC payload signing to Security section (defense-in-depth)
- Wrap MCP payload in mcp_request container for mutating webhooks
- Add mcp_version field to webhook request formats
- Clarify vMCP context fields (server_name vs backend_server)
- Add signing_secret_ref to configuration examples
- Promote HMAC signing to Phase 1, remove from Phase 3

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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Signed-off-by: Juan Antonio Osorio <ozz@stacklok.com>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: JAORMX