[NLP] Support the different mask tokens used by NLP models for Fill Mask

[maxhniebergall]

Adds the specification for the new mask token field added in elastic/elasticsearch#97453.

[droberts195]

LGTM

[github-actions]

Following you can find the validation results for the APIs you have changed.

API	Status	Request	Response
ml.clear_trained_model_deployment_cache	⚪	Missing test	Missing test
ml.close_job	🟢	62/62	61/61
ml.delete_calendar_event	🟢	4/4	4/4
ml.delete_calendar_job	🟢	3/3	3/3
ml.delete_calendar	🟢	5/5	5/5
ml.delete_data_frame_analytics	🟢	2/2	2/2
ml.delete_datafeed	🟢	3/3	3/3
ml.delete_expired_data	🟢	5/5	5/5
ml.delete_filter	🟢	27/27	27/27
ml.delete_forecast	🟢	3/3	3/3
ml.delete_job	🟢	47/47	47/47
ml.delete_model_snapshot	🟢	2/2	2/2
ml.delete_trained_model_alias	🟢	3/3	3/3
ml.delete_trained_model	🟢	3/3	3/3
ml.estimate_model_memory	🟢	16/16	16/16
ml.evaluate_data_frame	🟢	22/22	22/22
ml.explain_data_frame_analytics	🟢	7/7	7/7
ml.flush_job	🟢	15/15	15/15
ml.forecast	🟢	1/1	1/1
ml.get_buckets	🟢	14/14	14/14
ml.get_calendar_events	🟢	17/17	17/17
ml.get_calendars	🟢	17/17	17/17
ml.get_categories	🟢	12/12	12/12
ml.get_data_frame_analytics_stats	🟢	12/12	12/12
ml.get_data_frame_analytics	🟢	17/17	17/17
ml.get_datafeed_stats	🟢	27/27	27/27
ml.get_datafeeds	🟢	20/20	20/20
ml.get_filters	🟢	13/13	13/13
ml.get_influencers	🟢	11/11	11/11
ml.get_job_stats	🟢	31/31	31/31
ml.get_jobs	🟢	31/31	31/31
ml.get_memory_stats	⚪	Missing test	Missing test
ml.get_model_snapshot_upgrade_stats	🟢	3/3	3/3
ml.get_model_snapshots	🟢	18/18	18/18
ml.get_overall_buckets	🟢	16/16	15/15
ml.get_records	🟢	8/8	8/8
ml.get_trained_models_stats	🟢	12/12	12/12
ml.get_trained_models	🔴	21/27	27/27
ml.infer_trained_model	⚪	Missing test	Missing test
ml.info	🟢	10/10	10/10
ml.open_job	🟢	83/83	83/83
ml.post_calendar_events	🟢	21/21	21/21
ml.post_data	🔴	8/10	13/17
ml.preview_data_frame_analytics	🟢	3/3	3/3
ml.preview_datafeed	🔴	10/16	16/16
ml.put_calendar_job	🔴	11/12	12/12
ml.put_calendar	🟢	135/135	135/135
ml.put_data_frame_analytics	🟢	33/33	33/33
ml.put_datafeed	🔴	70/71	71/71
ml.put_filter	🟢	27/27	27/27
ml.put_job	🔴	218/226	224/224
ml.put_trained_model_alias	🟢	9/9	9/9
ml.put_trained_model_definition_part	⚪	Missing test	Missing test
ml.put_trained_model_vocabulary	⚪	Missing test	Missing test
ml.put_trained_model	🟢	5/5	5/5
ml.reset_job	🟢	2/2	2/2
ml.revert_model_snapshot	🟢	2/2	2/2
ml.set_upgrade_mode	🟢	6/6	6/6
ml.start_data_frame_analytics	🟢	1/1	1/1
ml.start_datafeed	🟢	24/24	24/24
ml.start_trained_model_deployment	⚪	Missing test	Missing test
ml.stop_data_frame_analytics	🟢	5/5	5/5
ml.stop_datafeed	🟢	17/17	17/17
ml.stop_trained_model_deployment	⚪	Missing test	Missing test
ml.update_data_frame_analytics	🟢	2/2	2/2
ml.update_datafeed	🟢	7/7	7/7
ml.update_filter	🟢	3/3	3/3
ml.update_job	🔴	4/5	5/5
ml.update_model_snapshot	🟢	3/3	3/3
ml.update_trained_model_deployment	🟠	Missing type	Missing type
ml.upgrade_job_snapshot	🟢	3/3	3/3
ml.validate_detector	🟢	2/2	2/2
ml.validate	🟢	3/3	3/3

You can validate these APIs yourself by using the make validate target.