…tch capabilities of the correlation id middleware

* Added optional "header_key" param to asgi_correlation_id.extensions.celery.load_correlation_id
* Added optional "generator" param to asgi_correlation_id.extensions.celery.load_correlation_id
* Added optional "generator" param to asgi_correlation_id.extensions.celery.load_celery_current_and_parent_ids

…ilter and standardized the string trimming process for the filtering logic

* changed type from int to Optional[int] for optional parameter 'uuid_length' in CeleryTracingIdsFilter
* changed default value from 32 to None for optional parameter 'uuid_length' in CeleryTracingIdsFilter
* created and utilized function `_trim_string` to standardize string trimming logic in filters
* added test to test string trimming for CeleryTracingIdsFilter
* added test to ensure the default behavior of the new filter matched the behavior of the old filter.  This assumes default generators are used

…he internally generated celery_id

* Added optional argument `use_internal_celery_task_id` to `load_celery_current_and_parent_ids`
* When `use_internal_celery_task_id` is set to True, the internal celery task ID will be used and the generator function will be ignored
* Updated README with information on this new argument