sql: add default_value and is_overridden to cluster settings

Fixes https://cockroachlabs.atlassian.net/browse/CRDB-28519

Previosly, there was no easy way to see default values for
cluster settings. This commit add the column for `default_value`
and `is_overridden` to `crdb_internal.cluster_settings` and
the `show cluster settings` command.

Release note (sql change): Add columns `default_value` and
`is_overridden` to `crdb_internal.cluster_settings` and
`show cluster settings` command..

pkg/sql/crdb_internal.go

@@ -2063,7 +2063,9 @@ CREATE TABLE crdb_internal.cluster_settings (
   value         STRING NOT NULL,
   type          STRING NOT NULL,
   public        BOOL NOT NULL, -- whether the setting is documented, which implies the user can expect support.
-  description   STRING NOT NULL
+  description   STRING NOT NULL,
+  default_value STRING NOT NULL,
+  is_overridden BOOL NOT NULL
 )`,
 	populate: func(ctx context.Context, p *planner, _ catalog.DatabaseDescriptor, addRow func(...tree.Datum) error) error {
 		if hasPriv, err := func() (bool, error) {
@@ -2110,12 +2112,19 @@ CREATE TABLE crdb_internal.cluster_settings (
 			strVal := setting.String(&p.ExecCfg().Settings.SV)
 			isPublic := setting.Visibility() == settings.Public
 			desc := setting.Description()
+			defaultVal, err := setting.DecodeToString(setting.EncodedDefault())
+			if err != nil {
+				defaultVal = setting.EncodedDefault()
+			}
+			isOverridden := strVal != defaultVal
 			if err := addRow(
 				tree.NewDString(k),
 				tree.NewDString(strVal),
 				tree.NewDString(setting.Typ()),
 				tree.MakeDBool(tree.DBool(isPublic)),
 				tree.NewDString(desc),
+				tree.NewDString(defaultVal),
+				tree.MakeDBool(tree.DBool(isOverridden)),
 			); err != nil {
 				return err
 			}

pkg/sql/delegate/show_all_cluster_settings.go

@@ -77,12 +77,12 @@ func (d *delegator) delegateShowClusterSettingList(
 
 	if stmt.All {
 		return d.parse(
-			`SELECT variable, value, type AS setting_type, public, description
+			`SELECT variable, value, type AS setting_type, public, description, default_value, is_overridden
        FROM   crdb_internal.cluster_settings`,
 		)
 	}
 	return d.parse(
-		`SELECT variable, value, type AS setting_type, description
+		`SELECT variable, value, type AS setting_type, description, default_value, is_overridden
      FROM   crdb_internal.cluster_settings
      WHERE  public IS TRUE`,
 	)

pkg/sql/logictest/testdata/logic_test/cluster_settings

@@ -78,6 +78,12 @@ WHERE variable IN ('sql.defaults.default_int_size')
 ----
 sql.defaults.default_int_size  4
 
+query TTTB
+SELECT variable, value, default_value, is_overridden FROM [SHOW ALL CLUSTER SETTINGS]
+WHERE variable IN ('sql.defaults.default_int_size')
+----
+sql.defaults.default_int_size  4 4 f
+
 user root
 
 statement ok