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](rtpsw@802674b) 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>

go/arrow/schema.go

@@ -194,12 +194,13 @@ func (sc *Schema) WithEndianness(e endian.Endianness) *Schema {
 func (sc *Schema) Endianness() endian.Endianness { return sc.endianness }
 func (sc *Schema) IsNativeEndian() bool          { return sc.endianness == endian.NativeEndian }
 func (sc *Schema) Metadata() Metadata            { return sc.meta }
-func (sc *Schema) Fields() []Field { 
+func (sc *Schema) Fields() []Field {
 	fields := make([]Field, len(sc.fields))
 	copy(fields, sc.fields)
 	return fields
 }
-func (sc *Schema) Field(i int) Field             { return sc.fields[i] }
+func (sc *Schema) Field(i int) Field { return sc.fields[i] }
+func (sc *Schema) NumFields() int    { return len(sc.fields) }
 
 func (sc *Schema) FieldsByName(n string) ([]Field, bool) {
 	indices, ok := sc.index[n]

go/arrow/schema_test.go

@@ -342,7 +342,7 @@ func TestSchemaAddField(t *testing.T) {
 	if got, want := len(s.Fields()), 3; got != want {
 		t.Fatalf("invalid number of fields. got=%d, want=%d", got, want)
 	}
-	got, want := s.Field(2), Field{Name: "f3", Type: PrimitiveTypes.Int32};
+	got, want := s.Field(2), Field{Name: "f3", Type: PrimitiveTypes.Int32}
 	if !got.Equal(want) {
 		t.Fatalf("invalid field: got=%#v, want=%#v", got, want)
 	}
@@ -462,3 +462,19 @@ func TestSchemaEqual(t *testing.T) {
 		})
 	}
 }
+
+func TestSchemaNumFields(t *testing.T) {
+	s := NewSchema([]Field{
+		{Name: "f1", Type: PrimitiveTypes.Int32},
+		{Name: "f2", Type: PrimitiveTypes.Int64},
+	}, nil)
+
+	assert.Equal(t, 2, s.NumFields())
+
+	var err error
+	s, err = s.AddField(2, Field{Name: "f3", Type: PrimitiveTypes.Int32})
+	assert.NoError(t, err)
+
+	assert.Equal(t, 3, s.NumFields())
+	assert.Equal(t, s.NumFields(), len(s.Fields()))
+}