sql: add sequence option info for identity columns under information_…

…schema

Previously, for a column created with the `GENERATED ... AS IDENTITY
(seq_options)` syntax, the info for the sequence option is not saved in the
information schema.

This commit is to fix it. We parse the sequence options saved as a string in the
descriptor, so that it's much easier to extract specific option such as
sequence's start value or increment size.

To make sure that we get the same sequence option to generate
the sequence, we reuse `assignSequenceOptions()` by breaking it into several
helper functions.

fixes #82064

Release note (sql): add sequence option info for identity columns under
information_schema

pkg/sql/catalog/schemaexpr/column.go

@@ -107,7 +107,7 @@ func FormatColumnForDisplay(
 				f.WriteString(" GENERATED BY DEFAULT AS IDENTITY")
 			}
 			if col.HasGeneratedAsIdentitySequenceOption() {
-				seqOpt := col.GetGeneratedAsIdentitySequenceOption()
+				seqOpt := col.GetGeneratedAsIdentitySequenceOptionStr()
 				s := formatGeneratedAsIdentitySequenceOption(seqOpt)
 				f.WriteString(s)
 			}

pkg/sql/catalog/table_elements.go

@@ -367,9 +367,14 @@ type Column interface {
 	// `GENERATED AS IDENTITY` column.
 	HasGeneratedAsIdentitySequenceOption() bool
 
+	// GetGeneratedAsIdentitySequenceOptionStr returns the string representation
+	// of the column's `GENERATED AS IDENTITY` sequence option if it exists, empty
+	// string otherwise.
+	GetGeneratedAsIdentitySequenceOptionStr() string
+
 	// GetGeneratedAsIdentitySequenceOption returns the column's `GENERATED AS
 	// IDENTITY` sequence option if it exists, empty string otherwise.
-	GetGeneratedAsIdentitySequenceOption() string
+	GetGeneratedAsIdentitySequenceOption() *descpb.TableDescriptor_SequenceOpts
 }
 
 // ConstraintToUpdate is an interface around a constraint mutation.