Address WebSocket flexibility feedback and add plugin to main README

This commit addresses reviewer @h3xxit's feedback that the WebSocket implementation was "too restrictive" by implementing maximum flexibility to work with ANY WebSocket endpoint.

Key Changes:
- **Flexible Message Templating**: Added `message` field (Union[str, Dict[str, Any]]) with ${arg_name} placeholder support
  - Dict templates: Support structured messages like JSON-RPC, chat protocols
  - String templates: Support text-based protocols like IoT commands
  - No template (default): Sends arguments as-is in JSON for maximum compatibility

- **Flexible Response Handling**: Added `response_format` field (Optional["json", "text", "raw"])
  - No format (default): Returns raw response without processing
  - Works with any WebSocket response structure

- **Removed Restrictive Fields**:
  - Removed `request_data_format`, `request_data_template`, `message_format`
  - No longer enforces specific request/response structure

- **Implementation**:
  - Added `_substitute_placeholders()` method for recursive template substitution
  - Updated `_format_tool_call_message()` to use template or send args as-is
  - Updated `call_tool()` to return raw responses by default

- **Testing**: Updated all 9 tests to reflect new flexibility approach

- **Documentation**:
  - Updated README to emphasize "maximum flexibility" principle
  - Added examples showing no template, dict template, and string template usage
  - Added WebSocket entry to main README plugin table

Philosophy: "Talk to as many WebSocket endpoints as possible" - UTCP should adapt to existing endpoints, not require endpoints to adapt to UTCP.

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

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

Author: alimoradi296