Plugin system for custom auth patterns

[kobzevvv]

Why: Some APIs use exotic auth mechanisms (AWS Sig v4, Digest auth, HMAC, custom signing). Instead of building each one into core, provide a plugin interface.

Scope:

• Plugin interface: receive credential data, return modified request (headers, URL, body)
• Load plugins from ~/.vault-mcp/plugins/
• Built-in plugins for common patterns

Use cases:

• AWS Sig v4 as a plugin
• MongoDB Atlas Digest auth
• Custom HMAC-signing APIs

Priority: Medium — enables community contributions