Adding webhook implementation

[asachs01]

#Overview

This PR implements a complete webhook system for the Autotask API client, including event handling and webhook verification. It also fixes pagination parameter handling issues that were causing incorrect results.

#Changes

##Webhook Implementation

• Added webhook handler registration system allowing clients to register handlers for specific event types
• Implemented webhook event handling with proper routing to registered handlers
• Added support for webhook signature verification using HMAC-SHA256
• Created a new SetWebhookSecret method to configure the webhook verification secret
• Implemented proper HTTP status code responses for webhook requests

##Pagination Fixes

• Fixed an issue where pagination parameters were incorrectly included in the filter string
• Updated the FetchPage function to properly set page parameters in query parameters
• Ensured distinct results are displayed for each page

##Documentation and Examples

• Added comprehensive webhook example in examples/webhook/
• Created detailed README for the webhook example explaining setup and usage
• Updated main README with webhook usage information
• Added tests for webhook functionality

##Testing

• Added unit tests for webhook handler registration
• Added tests for webhook event handling
• Added tests for webhook signature verification
• Manually tested pagination fixes with real API calls

##Breaking Changes

None. All changes are backward compatible.

##Future Work

• Add support for batch webhook processing
• Implement webhook delivery retry mechanism
• Add webhook event history tracking