Add proposal for dynamic webhook middleware

Introduces a webhook middleware system that enables external services to
participate in the MCP request pipeline through validating and mutating
webhooks, inspired by Kubernetes admission webhooks.

Key features:
- Validating webhooks for accept/deny decisions
- Mutating webhooks for request transformation
- Multiple webhook support with sequential execution
- File-based CLI configuration
- Kubernetes MCPWebhookConfig CRD
- TLS/mTLS support for webhook authentication
- Configurable failure policies (fail-open/fail-closed)
- Full observability (metrics, audit logs, tracing)

Benefits:
- Organizations can extend ToolHive without code changes
- Integrate with external systems (policy engines, SIEM, HR, CMDB)
- Enforce approval workflows and rate limits
- Request enrichment and PII redaction
- Consistent with ToolHive architecture and patterns

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: JAORMX