Fix: use Rust idiomatic capitalization for newly added DataType enums

[Kikkon]

In my previous commit #931 , I think the handling of case for the PostgreSQL type aliases is incorrect. We should modify it to first letter capitalized

[coveralls]

Pull Request Test Coverage Report for Build 5716237483

• 4 of 6 (66.67%) changed or added relevant lines in 3 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage remained the same at 87.051%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/ast/data_type.rs	0	2	0.0%

Files with Coverage Reduction	New Missed Lines	%
src/parser.rs	1	82.98%

Totals
Change from base Build 5679999493:	0.0%
Covered Lines:	15516
Relevant Lines:	17824

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💛 - Coveralls

[Kikkon]

This change is related to PostgreSQL alias types. ： apache/datafusion#7156
Maybe we can merge this, then publish a release version. @alamb

[alamb]

Makes sense to me -- thank you @Kikkon