[YSQL] foreign key constraints are not supported on foreign tables

[tedyu]

Jira Link: DB-1033

Description

yugabyte=# CREATE TABLE ref_table (id integer PRIMARY KEY);
CREATE TABLE
yugabyte=# CREATE FOREIGN TABLE ft2 (
yugabyte(#     c1 integer OPTIONS ("param 1" 'val1') REFERENCES ref_table (id),
yugabyte(#     c2 text OPTIONS (param2 'val2', param3 'val3'),
yugabyte(#     c3 date
yugabyte(# ) SERVER s0 OPTIONS (delimiter ',', quote '"', "be quoted" 'value');
WARNING:  'foreign data wrapper' is a beta feature!
LINE 1: CREATE FOREIGN TABLE ft2 (
        ^
HINT:  To suppress this warning, set the 'ysql_beta_feature_foreign data wrapper' yb-tserver gflag to true.
(Set 'ysql_beta_features' yb-tserver gflag to true to suppress the warning for all beta features.)
ERROR:  foreign key constraints are not supported on foreign tables
LINE 2:     c1 integer OPTIONS ("param 1" 'val1') REFERENCES ref_tab...
                                                  ^