Support SHOW FUNCTIONS

[joocer]

Add support for the Trino-specific SHOW FUNCTIONS statement (https://trino.io/docs/current/sql/show-functions.html?highlight=show).

I've used SHOW COLUMNS as the template.

There doesn't appear to be any Trino or Presto-specific sections in the code to align this too.

Relates to issue #616 , although I wouldn't say this resolves that issue.

[joocer]

Thanks - I believe I've addressed the CI quality check errors.

[coveralls]

Pull Request Test Coverage Report for Build 3122548427

• 15 of 15 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 85.608%

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Totals
Change from base Build 3090626203:	0.02%
Covered Lines:	9767
Relevant Lines:	11409

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💛 - Coveralls

[Dandandan]

Thanks @joocer