[Variant] Support nested lists and object lists (#7740)

# Which issue does this PR close?

- Closes #7696

# Rationale for this change

Heavily refactors `VariantBuilder`, `ObjectBuilder`, and `ListBuilder`
to allow nested list and object building.

Now, `ObjectBuilder` and `ListBuilder` have their own `VariantBuffer`,
an intermediate buffer that gets written to upon every call to `insert`
or `append_value`. Only when `finish` is called will the builder flush
the intermediate buffer to its parent buffer.

`VariantBuilder` was split to hold `VariantBuffer` and
`FieldMetadataDictionary` to better simplify and separate the nesting
logic.

Author: friendlymatthew