host_url option to CLI and Databricks validator

[sean-sinclair]

This pull request adds support for validating Databricks Personal Access Tokens, including a new validator, CLI integration, documentation, and comprehensive tests. The main changes are grouped below.

Databricks Token Validation Support:

• Introduced the DatabricksTokenChecker class in validators/databricks_token.py, which validates Databricks Personal Access Tokens against a specified workspace URL, supporting both CLI flag and environment variable configuration for the host URL.
• Updated the CLI (cli.py) to accept a new --host-url option, passing it through to validators and ensuring the host URL is available in all relevant commands (check_file, check_github, validate). [1] [2] [3] [4] [5]
• Modified the Checker base class to accept and handle the host_url parameter, stripping trailing slashes and documenting its usage.

Documentation Updates:

• Added Databricks token type to the supported secrets table and included example usage and instructions for Databricks token validation in the README.md. [1] [2]

Testing Enhancements:

• Added tests for the new validator, including host URL configuration, environment variable fallback, and error handling when the host is not set (test_validators.py). Also updated validator metadata tests to include the new Databricks validator. [1] [2] [3]
• Ensured the new validator is registered and discoverable in the registry tests.