[Variant] Add Variant::as_object and Variant::as_list

[alamb]

Which issue does this PR close?

• part of [EPIC] [Parquet] Implement Variant type support in Parquet #6736

Rationale for this change

• While reviewing @friendlymatthew 's PR [Variant] Support nested lists and object lists #7740 I found that the code to get the Variant object was awkward

I think that an accessor is similar to the existing as_null, as_i32, etc APIs.

What changes are included in this PR?

1. Add Variant::as_object and Variant::as_list

Are there any user-facing changes?

New API (and docs with tests)

[crepererum]

I'm merging this to unblock #7740.