GH-46087: [FlightSQL] Allow returning column remarks in FlightSQL's CommandGetTables (#361)

### Rationale for this change

See apache/arrow#46110

### What changes are included in this PR?

A new column metadata value

### Are these changes tested?

No; there seem to be no tests that'd verify that `ColumnMetadata` works;
should I add unit tests specifically for the new methods?

### Are there any user-facing changes?

Yes, two new methods

Author: mateuszrzeszutek

arrow/flight/flightsql/column_metadata.go

@@ -50,6 +50,7 @@ const (
 	IsCaseSensitiveKey = "ARROW:FLIGHT:SQL:IS_CASE_SENSITIVE"
 	IsReadOnlyKey      = "ARROW:FLIGHT:SQL:IS_READ_ONLY"
 	IsSearchableKey    = "ARROW:FLIGHT:SQL:IS_SEARCHABLE"
+	RemarksKey         = "ARROW:FLIGHT:SQL:REMARKS"
 )
 
 // ColumnMetadata is a helper object for managing and querying the
@@ -130,6 +131,10 @@ func (c *ColumnMetadata) IsSearchable() (bool, bool) {
 	return c.findBoolVal(IsSearchableKey)
 }
 
+func (c *ColumnMetadata) Remarks() (string, bool) {
+	return c.findStrVal(RemarksKey)
+}
+
 // ColumnMetadataBuilder is a convenience builder for constructing
 // sql column metadata using the expected standard metadata keys.
 // All methods return the builder itself so it can be chained
@@ -215,3 +220,9 @@ func (c *ColumnMetadataBuilder) IsSearchable(v bool) *ColumnMetadataBuilder {
 	c.vals = append(c.vals, boolToStr(v))
 	return c
 }
+
+func (c *ColumnMetadataBuilder) Remarks(remarks string) *ColumnMetadataBuilder {
+	c.keys = append(c.keys, RemarksKey)
+	c.vals = append(c.vals, remarks)
+	return c
+}

arrow/internal/flight_integration/scenario.go

@@ -2228,6 +2228,7 @@ func getQuerySchema() *arrow.Schema {
 				IsSearchable(true).
 				CatalogName("catalog_test").
 				Precision(100).
+				Remarks("test column").
 				Build().Data}}, nil)
 }
 
@@ -2242,6 +2243,7 @@ func getQueryWithTransactionSchema() *arrow.Schema {
 				SchemaName("schema_test").
 				IsSearchable(true).
 				CatalogName("catalog_test").
+				Remarks("test column").
 				Precision(100).Build().Data}}, nil)
 }