[Variant] Define basic convenience methods for variant pathing

[scovich]

Which issue does this PR close?

Part of

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

Rationale for this change

An expected use case for variant pathing (e.g. a future variant_get) would be to request a field from a variant value that is expected to be an object, or an element from a variant value that is expected to be an array. Those methods are currently missing.

What changes are included in this PR?

Define Variant::get_object_field and Variant::get_array_element methods that do what they say (returning None if anything mismatches).

Are these changes tested?

New doc tests for the methods.

Are there any user-facing changes?

New public methods on Variant enum.

[alamb]

Looks great to me -- thank you @scovich