Disambiguate CREATE ROLE ... USER and GROUP

[alamb]

Rather than assume ROLE and USER are synonyms in the parser, let's parse them separately and allow upstream crates to treat them the same if they desire

Per comment https://github.com/sqlparser-rs/sqlparser-rs/pull/598/files#r981277941 on contribution from @blx

[alamb]

cc @blx

[coveralls]

Pull Request Test Coverage Report for Build 3136427473

• 27 of 40 (67.5%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.05%) to 85.501%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tests/sqlparser_postgres.rs	11	12	91.67%
src/ast/mod.rs	4	6	66.67%
src/parser.rs	12	22	54.55%

Totals
Change from base Build 3136407158:	-0.05%
Covered Lines:	10102
Relevant Lines:	11815

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💛 - Coveralls

[blx]

This seems more sensible and flexible 👍