Enhancement: Support custom Azure Functions Core Tools installation paths

Is your feature request related to a problem? Please describe.
Currently, the extension only looks for the func CLI in the system PATH. Users who have Azure Functions Core Tools installed in custom locations or use multiple versions cannot use the extension without modifying their system PATH.

Describe the solution you'd like
Add support for custom func CLI paths through:

• VS Code setting: azureFunctions.funcPath to specify custom func executable path
• Automatic detection in common installation locations
• UI prompt to browse for func executable when not found in PATH

Describe alternatives you've considered

• Requiring users to modify system PATH (current limitation)
• Only supporting package manager installations (doesn't cover all use cases)
• Using bundled Core Tools (increases extension size significantly)

Additional context

• Related to issue #4601
• Current implementation only checks system PATH for func CLI detection
• Users with custom installations or multiple versions face installation issues
• Enhancement would improve developer experience for complex development environments

Implementation considerations

• Should integrate with existing Core Tools detection logic
• Need to validate custom paths point to working func installations
• Consider version validation for custom paths
• Should fallback to PATH detection if custom path not configured
• May need updates to debugging and task provider configurations

Start by locating the existing Azure Functions Core Tools detection logic and review how debugging and task provider configurations consume the detected executable. Define the setting, common-location detection, browse prompt, path validation, version handling, and PATH fallback, then verify that custom installations work across those integrations.