arrow

GH-36384: [Go] Schema: NumFields (#36365)



### Rationale for this change

Previously if one wanted to iterate over the fields in the schema you would call the `Fields()` function and just iterate over the slice. However, due to [this commit](https://github.com/rtpsw/arrow/commit/802674b73c94c84388a6775b424ebe4f6e04274e) there is now an allocation and copy that happens when that's called. So to iterate over the fields without allocations one now must use the `Field(i int)` method; however that means a user must already know exactly how many fields are in the schema which isn't possible today.

This adds a simple `NumFields() int` method that returns the number of fields in a schema to allow a user to iterate over all the fields without having to copy them. 

### What changes are included in this PR?

Expose the number of fields in a schema for iteration over fields.

Single function added `NumFields() int` to schema

### Are these changes tested?

N/A

### Are there any user-facing changes?

Yes this is a new API

* Closes: #36384

Authored-by: thorfour <me@thor-hansen.com>
Signed-off-by: Matt Topol <zotthewizard@gmail.com>