Fix YAML validation for ElasticsearchDocumentStore.custom_query

[ZanSara]

Related Issue(s): #2754

Proposed changes:

• Add an exception for this specific field in the validation code.
• Introduce the possibility to list JSON fields by name and have them validated as JSON instead of strings
• CATCH: the fields are validated by name, not by which component is trying to load them. The implementation is not very neat, better ideas are welcome.

Pre-flight checklist

• I have read the contributors guidelines
• I have enabled actions on my fork
• If this is a code change, I added tests or updated existing ones
• If this is a code change, I updated the docstrings

[julian-risch]

LGTM! 👍 It's good to have a first fix for this but I also agree that the solution is hacky because of the need to have matching names. Further, if a user wants to add a custom node that has a parameter that accepts JSON then this becomes more complex. For now, I found only this single JSON field "custom_query" when skimming through all our nodes and "custom_query" is relatively important to allow use cases that otherwise cannot be handled with the standard queries (custom filtering, etc.). So let's merge this fix! 👍