[yugabyte#27495] YSQL: add ysql_yb_enable_cbo tserver flag

Summary:
Add tserver GFlag counter part of yb_enable_cbo, a GUC parameter introduced by yugabyte#26868.
Jira: DB-17048

Test Plan: Manual test with both valid and invalid values, jenkins

Reviewers: hsunder, mihnea

Reviewed By: hsunder

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D44518

Author: mtakahar

src/yb/yql/pgwrapper/pg_wrapper.cc

@@ -262,6 +262,10 @@ DEFINE_RUNTIME_PG_PREVIEW_FLAG(bool, yb_enable_optimizer_statistics, false,
 DEFINE_RUNTIME_PG_PREVIEW_FLAG(bool, yb_enable_base_scans_cost_model, false,
     "Enable cost model enhancements");
 
+DEFINE_RUNTIME_PG_FLAG(string, yb_enable_cbo, "legacy_mode",
+    "YSQL cost-based optimizer mode. Allowed values are 'legacy_mode', 'legacy_stats_mode', "
+    "'off', and 'on'");
+
 DEFINE_RUNTIME_PG_FLAG(uint64, yb_fetch_row_limit, 1024,
     "Maximum number of rows to fetch per scan.");
 
@@ -528,6 +532,10 @@ static bool ValidateDocumentDB(const char* flag_name, bool value) {
 
 DEFINE_validator(ysql_enable_documentdb, &ValidateDocumentDB);
 
+// Keep the value list in sync with `yb_cost_model_options` in `guc.c`.
+DEFINE_validator(ysql_yb_enable_cbo,
+    FLAG_IN_SET_VALIDATOR("off", "on", "legacy_mode", "legacy_stats_mode"));
+
 namespace {
 // Append any Pg gFlag with non default value, or non-promoted AutoFlag
 void AppendPgGFlags(vector<string>* lines) {