sql: add unimplemented errors for trigram indexes and functions

[awoods187]

Similarity

root@127.0.0.1:60727/movr> SELECT similarity('word', 'two words');
ERROR: unknown function: similarity()
SQLSTATE: 42883

Using show_trgm() function:

#
root@127.0.0.1:60727/movr> SELECT show_trgm('hello');
ERROR: unknown function: show_trgm()
SQLSTATE: 42883

Word_similarity

root@127.0.0.1:60727/movr> SELECT word_similarity('word', 'two words');
ERROR: unknown function: word_similarity()
SQLSTATE: 42883

Strict_world_similarity

root@127.0.0.1:60727/movr> SELECT strict_word_similarity('word', 'two words');
ERROR: unknown function: strict_word_similarity()
SQLSTATE: 42883

Creating an index:

root@127.0.0.1:60727/movr> CREATE INDEX CONCURRENTLY index_vehicles_on_vehicle_name_trigram
ON vehicles
USING gin (ext gin_trgm_ops);
invalid syntax: statement ignored: at or near "gin_trgm_ops": syntax error
SQLSTATE: 42601
DETAIL: source SQL:
CREATE INDEX CONCURRENTLY index_vehicles_on_vehicle_name_trigram
ON vehicles
USING gin (ext gin_trgm_ops)
               ^
HINT: try \h CREATE INDEX

or

[email protected]:60727/movr> CREATE INDEX trgm_idx ON test_trgm USING GIST (t gist_trgm_ops);
invalid syntax: statement ignored: at or near "gist": syntax error: unimplemented: this syntax
SQLSTATE: 0A000
DETAIL: source SQL:
CREATE INDEX trgm_idx ON test_trgm USING GIST (t gist_trgm_ops)
                                         ^
HINT: You have attempted to use a feature that is not yet implemented.

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We also need:

show_limit()
set_limit(real)

https://www.postgresql.org/docs/12/pgtrgm.html

Relates to #41285