Add per-server service authentication

[dgellow]

Previously, authentication was configured globally at the proxy level. This meant all servers had to use the same auth method.

Now each MCP server can have its own authentication. This is useful when you have different services that need different auth - for example, one service uses API keys while another uses basic auth.

Example config:

{
  "proxy": {
    "baseURL": "http://localhost:8080",
    "addr": ":8080"
  },
  "mcpServers": {
    "postgres": {
      "transportType": "stdio",
      "command": "docker",
      "args": ["run", "-i", "mcp/postgres"],
      "serviceAuths": [
        {
          "type": "bearer",
          "tokens": ["api-key-123", "api-key-456"]
        }
      ]
    },
    "notion": {
      "transportType": "sse",
      "url": "https://notion-mcp.example.com/sse",
      "serviceAuths": [
        {
          "type": "basic",
          "username": "admin",
          "password": {"$env": "NOTION_PASSWORD"}
        }
      ]
    }
  }
}

When a server requires user tokens, each service auth can specify its own:

"serviceAuths": [
  {
    "type": "bearer",
    "tokens": ["service-key"],
    "userToken": {"$env": "NOTION_USER_TOKEN"}
  }
]

[dgellow]

LGTM