added ability to parse extension to parse_comment inside postgres dialect

[MaxwellKnight]

Hi guys,
First of all i'd say that i'm new to contributing to open source so if I've made a mistake i'm deeply sorry.
Also I made sure to write the relevant tests and I ran cargo test, cargo fmt and cargo clippy and it said in the README.
I added the ability to parse COMMENT ON EXTENSION statements to postgres dialect,
I've been using this crate for a while now and I saw that I get a parser error when I have these kind of statements.

[iffyio]

LGTM! cc @alamb

[alamb]

First of all i'd say that i'm new to contributing to open source so if I've made a mistake i'm deeply sorry.

No need to apologize! Thank you for the contribution -- we appreciate everyone's efforts (it is how we keep these things going)

[alamb]

Thanks @MaxwellKnight and @iffyio

[coveralls]

Pull Request Test Coverage Report for Build 11131556477

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 14 of 16 (87.5%) changed or added relevant lines in 3 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.001%) to 89.302%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tests/sqlparser_postgres.rs	10	12	83.33%

Files with Coverage Reduction	New Missed Lines	%
src/dialect/postgresql.rs	1	69.47%

Totals
Change from base Build 11111316639:	-0.001%
Covered Lines:	29951
Relevant Lines:	33539

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💛 - Coveralls