[Variant] VariantArray field API naming

[sdf-jkl]

Which issue does this PR close?

• Closes [Variant] VariantArray field API naming #10093.

Rationale for this change

check issue

What changes are included in this PR?

• Rename existing _field APIs that return &ArrayRef to _column
• Add new _field APIs that return &FieldRef and tests for them

Are these changes tested?

• Yes, unit tests

Are there any user-facing changes?

• Yes, breaking API name change.

[sdf-jkl]

Now that I look at it the new _field APIs would only be used for tests in #10015.

  // before reuse StructArray::field_by_name (#10110), returns Option<&FieldRef>
  let typed_value_field = variant_array.inner().field_by_name("typed_value").unwrap();
  
  // new — the new VariantArray sugar, returns Option<&FieldRef> (no .inner(), no magic string)
  let typed_value_field = variant_array.typed_value_field().unwrap();

Is it worth introducing for 4 calls in tests?

[sdf-jkl]

@alamb @scovich @klion26 PTAL