GH-35256: [Go] Add ToMap to Metadata (#35257)

### Rationale for this change

### What changes are included in this PR?

### Are these changes tested?

### Are there any user-facing changes?

* Closes: #35256
* Closes: #35256

Authored-by: Yevgeny Pats <16490766+yevgenypats@users.noreply.github.com>
Signed-off-by: Matthew Topol <zotthewizard@gmail.com>

go/arrow/schema.go

@@ -66,6 +66,13 @@ func MetadataFrom(kv map[string]string) Metadata {
 func (md Metadata) Len() int         { return len(md.keys) }
 func (md Metadata) Keys() []string   { return md.keys }
 func (md Metadata) Values() []string { return md.values }
+func (md Metadata) ToMap() map[string]string {
+	m := make(map[string]string, len(md.keys))
+	for i := range md.keys {
+		m[md.keys[i]] = md.values[i]
+	}
+	return m
+}
 
 func (md Metadata) String() string {
 	o := new(strings.Builder)

go/arrow/schema_test.go

@@ -22,6 +22,7 @@ import (
 	"testing"
 
 	"github.com/apache/arrow/go/v12/arrow/endian"
+	"github.com/stretchr/testify/assert"
 )
 
 func TestMetadata(t *testing.T) {
@@ -99,6 +100,9 @@ func TestMetadata(t *testing.T) {
 			if got, want := tc.md.String(), tc.serialize; got != want {
 				t.Fatalf("invalid stringer: got=%q, want=%q", got, want)
 			}
+			if len(tc.kvs) != 0 {
+				assert.Equal(t, tc.kvs, md.ToMap())
+			}
 		})
 	}