Support for Snowflake dynamic pivot

[jmhain]

Snowflake just gained a nifty new feature called Dynamic Pivot. This adds parser support for it (+ another PIVOT option that I noticed we were missing support for in the process).

[coveralls]

Pull Request Test Coverage Report for Build 9167250918

Details

• 37 of 39 (94.87%) changed or added relevant lines in 4 files are covered.
• 4 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.02%) to 89.479%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/ast/query.rs	9	10	90.0%
src/parser/mod.rs	19	20	95.0%

Files with Coverage Reduction	New Missed Lines	%
tests/sqlparser_common.rs	1	89.47%
src/ast/query.rs	3	84.38%

Totals
Change from base Build 9166712366:	0.02%
Covered Lines:	24961
Relevant Lines:	27896

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💛 - Coveralls

[iffyio]

LGTM! cc @alamb

[alamb]

Thank you @jmhain and @iffyio