Implements DROP POLICY syntax for PostgreSQL

[git-hulk]

DROP POLICY [ IF EXISTS ] name ON table_name [ CASCADE | RESTRICT ]

For the documentation, please refer:

https://www.postgresql.org/docs/current/sql-createpolicy.html

This closes #1404.

[git-hulk]

ALTER POLICY will be supported in another dedicated PR.

[git-hulk]

cc @iffyio

[coveralls]

Pull Request Test Coverage Report for Build 11052714807

Details

• 48 of 54 (88.89%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 89.323%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/ast/mod.rs	11	13	84.62%
tests/sqlparser_common.rs	17	21	80.95%

Totals
Change from base Build 11038149323:	0.01%
Covered Lines:	29825
Relevant Lines:	33390

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💛 - Coveralls

[iffyio]

LGTM! cc @alamb

[alamb]

Thank you @iffyio and @git-hulk -- you are making this very easy!