[YSQL] replace JsonReader with new JsonDocument

[jasonyb]

Jira Link: DB-19803

Description

The JsonReader class is a very old class and poorly designed. It claims
that it has no choice but to expose rapidjson internals, but that's not
true. All the Extract* functions should be static as they have nothing
to do with member variables. The Extract* functions take pointers for
filling results rather than returning Result<...>s. Protobuf-related
functions don't need to be implemented here and can instead be defined
in a separate utility.

Replace the whole class with a brand new class JsonDocument.

Issue Type

kind/enhancement

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