Improve or throw proper error for COPY FROM in Citus foreign tables

COPY FROM doesn't work for Citus foreign tables. Currently it fails with this error coming from one of the worker nodes:
```SQL
-- steps to reproduce
SET citus.use_citus_managed_tables TO ON;
CREATE TABLE foreign_table_test (id integer NOT NULL, data text, a bigserial);
INSERT INTO foreign_table_test VALUES (1, 'text_test');
CREATE EXTENSION postgres_fdw;
CREATE SERVER foreign_server
FOREIGN DATA WRAPPER postgres_fdw
OPTIONS (host 'localhost', port :'master_port', dbname 'regression');
CREATE USER MAPPING FOR CURRENT_USER
SERVER foreign_server
OPTIONS (user 'postgres');
CREATE FOREIGN TABLE foreign_table (
id integer NOT NULL,
data text,
a bigserial
)
SERVER foreign_server
OPTIONS (schema_name 'public', table_name 'foreign_table_test');

-- COPY FROM doesn't work for Citus foreign tables
COPY foreign_table FROM stdin;
1 1foo 2
\.
ERROR: cannot PREPARE a transaction that has operated on postgres_fdw foreign tables
```

It is a not a precise error. We should catch this error earlier more precisely in `EnsureCopyCanRunOnRelation`, with a message like `Citus cannot COPY into foreign tables`.

Also, we should make COPY not initiate a 2PC if it only touches a single shard. That way, we avoid this error in many cases.

Discussed with @onderkalaci @marcocitus https://github.com/citusdata/citus/pull/7160#discussion_r1314488889
Prioritized for 12.1 release.