arrow

GH-35866: [Go] Provide a copy in `arrow.NestedType.Fields()` implementations (#35867)

### Rationale for this change

Data types should be left immutable after they've been constructed.

### What changes are included in this PR?

Provide a copy of fields in the following implementations:
* `*arrow.StructType.Fields()`
* `*arrow.unionType.Fields()`

### Are these changes tested?

`arrow.TestFieldsImmutability` was added to ensure the behavior.

### Are there any user-facing changes?

Now the fields for structs & unions will be immutable.

* Closes: #35866

Authored-by: candiduslynx <candiduslynx@gmail.com>
Signed-off-by: Matt Topol <zotthewizard@gmail.com>