chore(search): Simplify return field logic

[dranikpg]

1. Remove effectively dead code:

• SearchField never stores generated fields, so string_view is enough instead of StringOrView + View() + etc...
• SearchField::is_short_name is always true
• Remove SearchField's dead functions
• DocumentAccessor::SerializeDocument replaced Serialize(), but the latter was never removed. Remove and swap names back

2. Add ReturnOptionJson that extensively tests RETURN with json
3. Remove quotes for returning plain json fields (like RS)

[BagritsevichStepan]

I wanted to create a wrapper for the json class to avoid mixing up to_string and as_string, since it’s not clear that they return different string formats (and we also call them in another place). But it’s okay.