feat: add Mocker plugin for intercepting and mocking AI provider responses (#106)

# Add Mocker Plugin for Bifrost

This PR introduces a new Mocker plugin for Bifrost that allows intercepting and mocking AI provider responses. The plugin provides flexible rule-based mocking capabilities for testing, development, and simulation purposes.

## Features

- Rule-based mocking with priority ordering and probability controls
- Support for custom success and error responses
- Template-based responses with variable substitution
- Configurable latency simulation
- Comprehensive statistics tracking
- High-performance design optimized for benchmarking

## Implementation Details

- Added core plugin implementation with pre-compiled regex patterns for performance
- Implemented weighted response selection for simulating varied behaviors
- Added support for provider/model matching and message content regex matching
- Included detailed validation for configuration parameters
- Added atomic counters for high-performance statistics tracking
- Created comprehensive test suite with various mocking scenarios

## Documentation

- Added detailed README with configuration reference, examples, and best practices
- Included performance optimization guidelines and troubleshooting tips
- Provided comprehensive API documentation for all configuration options

The plugin is designed to be lightweight when disabled and highly performant for benchmarking scenarios, with minimal overhead in the no-match path.

Author: akshaydeo

docs/mcp.md

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+```text
 Error: SSE stream error: context canceled
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docs/plugins.md

@@ -476,7 +476,7 @@ func (p *CachePlugin) PostHook(ctx *context.Context, result *BifrostResponse, er
 
 Each plugin should be organized as follows:
 
-```
+```text
 plugins/
 └── your-plugin-name/
     ├── main.go           # Plugin implementation