Enable PARTITION BY feature for PostgreSQL while parsing the create table statement

[git-hulk]

This closes #1281.

PARTITION BY has been supported but wasn't enabled for the PostgreSQL dialect, this PR only enables it and adds a test case to make sure it works well.

[coveralls]

Pull Request Test Coverage Report for Build 9902843006

Details

• 47 of 52 (90.38%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.005%) to 89.184%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tests/sqlparser_postgres.rs	29	34	85.29%

Totals
Change from base Build 9864598743:	-0.005%
Covered Lines:	26832
Relevant Lines:	30086

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💛 - Coveralls

[iffyio]

LGTM! cc @alamb

[alamb]

Thanks @git-hulk and @iffyio