Add neon.lakebase_mode boolean GUC (#12714)

This GUC will become useful for temporarily disabling Lakebase-specific
features during the code merge.

Signed-off-by: Tristan Partin <tristan.partin@databricks.com>

pgxn/neon/neon.c

@@ -48,6 +48,7 @@
 PG_MODULE_MAGIC;
 void		_PG_init(void);
 
+bool lakebase_mode = false;
 
 static int  running_xacts_overflow_policy;
 static bool monitor_query_exec_time = false;
@@ -583,6 +584,16 @@ _PG_init(void)
 							"neon_superuser",
 							PGC_POSTMASTER, 0, NULL, NULL, NULL);
 
+	DefineCustomBoolVariable(
+							"neon.lakebase_mode",
+							"Is neon running in Lakebase?",
+							NULL,
+							&lakebase_mode,
+							false,
+							PGC_POSTMASTER,
+							0,
+							NULL, NULL, NULL);
+
 	/*
 	 * Important: This must happen after other parts of the extension are
 	 * loaded, otherwise any settings to GUCs that were set before the

pgxn/neon/neon.h

@@ -21,6 +21,7 @@ extern int	wal_acceptor_reconnect_timeout;
 extern int	wal_acceptor_connection_timeout;
 extern int	readahead_getpage_pull_timeout_ms;
 extern bool	disable_wal_prev_lsn_checks;
+extern bool	lakebase_mode;
 
 extern bool AmPrewarmWorker;