feat: REST handlers without UI - OpenApi support

[kkedziak-splunk]

One of two PR's. It allows to specify REST handlers not generated by UCC in globalConfig. Based on that, UCC will extend openapi.json with additional entries. The second feature will contain the second feature + docs. Nothing changes in existing addons. This feature introduces new options in global config.

[kkedziak-splunk]

Description temporarily changed for running the review bot.

[kkedziak-splunk]

Suggested pull request title: feat: add support for custom REST handlers without UI in openapi.json

Great work on implementing support for custom REST handler definitions in the global configuration! The code is well-organized and includes comprehensive test coverage. The feature will allow users to specify additional REST endpoints not generated by UCC, which will be included in the OpenAPI documentation.

The implementation could benefit from a few minor improvements:

1. In user_defined_rest_handlers.py, consider adding docstrings to the classes RestHandlerConfig and UserDefinedRestHandlers explaining their purpose and usage.

2. In schema.json, the pattern for name and endpoint fields could be more descriptive, e.g., "^[a-zA-Z][a-zA-Z0-9_-]*$" to ensure they start with a letter.

3. In rest_builder/user_defined_rest_handlers.py, consider moving the EAI_DEFAULT_PARAMETERS and EAI_DEFAULT_PARAMETERS_SPECIFIED constants to a separate configuration file since they might need to be modified in the future.

4. In openapi_generator/ucc_to_oas.py, add docstring for the new function __add_user_defined_paths to explain its purpose and parameters.

5. In global_config.py, consider adding a docstring for the parse_user_defined_handlers method explaining when it should be called and what it does.

Overall, this appears to be a solid implementation ready for merging after addressing these minor documentation improvements. The test coverage is particularly impressive, with both unit tests and integration tests verifying the functionality.

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