Fix and tune synonyms API

[swallez]

Follow-up to #2234

This PR fixes some modelling issues and refines the specification:

• Rename SynonymRuleOptionalId and SynonymRule to SynonymRule and SynonymRuleRead, respectively, and use the OverloadOf feature.
• Removed inheritance between SynonymRule variations, since a subclass should not override a property defined in its parent class (need to add a validation rule for this)
• Use Id instead of Name
• Use @server_default to capture default values
• Fix response definitions where _shards: ShardStatistics was not a the correct nesting level

[carlosdelest]

LGTM, thank you for the improvements! 💯

[github-actions]

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

API	Status	Request	Response
indices.reload_search_analyzers	🟢	2/2	2/2
synonyms.delete_synonym_rule	⚪	Missing test	Missing test
synonyms.delete_synonym	⚪	Missing test	Missing test
synonyms.get_synonym_rule	⚪	Missing test	Missing test
synonyms.get_synonym	⚪	Missing test	Missing test
synonyms.get_synonyms_sets	⚪	Missing test	Missing test
synonyms.put_synonym_rule	⚪	Missing test	Missing test
synonyms.put_synonym	⚪	Missing test	Missing test

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