Modifier support for Custom Datatype

[sunng87]

In PostgreSQL/PostGIS, when creating tables with geometry or geography data type, additional modifiers are allowed. In this example, we can specify geography(POINT, 4269) as data type, which indicates the column is geography point typed, and uses 4269 as its spatial reference system.

Back to sqlparser, at the moment we don't support Custom data type that comes with additional modifiers/arguments in form of (..). This patch add support for that. Since these data can be in any form like an identifier, a number or a string, here I convert all of them into String. Suggestions are welcomed.

Edit: In PostGIS this is called type modifiers.

[alamb]

LGTM -- thanks @sunng87

[coveralls]

Pull Request Test Coverage Report for Build 3286510718

• 26 of 28 (92.86%) changed or added relevant lines in 2 files are covered.
• 8 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.04%) to 85.949%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/parser.rs	22	24	91.67%

Files with Coverage Reduction	New Missed Lines	%
src/parser.rs	8	83.66%

Totals
Change from base Build 3285178552:	0.04%
Covered Lines:	10900
Relevant Lines:	12682

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💛 - Coveralls