[opensearch_client] Migrate schemas to pydantic

[simskij]

Upon some closer investigation, I noticed that this interface is still using JSON schema only. I suggest you have a stab at implementing the schema with pydantic before we merge this as this will likely uncover things you might like to change.

One change that comes to mind just by reading the spec is that secret_fields is a space-delimited string rather than a string array. I would change this for multiple reasons:

• Easier to work with (with pydantic deserialization, that is)
• And probably more importantly, JSON allows spaces in object keys, which could potentially become a problem 😝.

(and sorry for this becoming so long-lived, but I do think this is important enough to stall)

Originally posted by @simskij in #99 (review)