feat: Add APPROXIMATE function keyword support

[MazterQyou]

This PR adds support for APPROXIMATE keyword for functions. This is used by Redshift with some functions, such as COUNT. A related test is also added.
Additionally, this PR locks Rust toolchain to the one used by cubesql, and locks versions of dependencies to avoid CI deprecation notices in newer versions.

[coveralls]

Pull Request Test Coverage Report for Build 3588892612

• 30 of 35 (85.71%) changed or added relevant lines in 3 files are covered.
• 4 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+4.6%) to 89.798%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tests/sqlparser_postgres.rs	20	22	90.91%
src/parser.rs	8	11	72.73%

Files with Coverage Reduction	New Missed Lines	%
src/parser.rs	4	82.96%

Totals
Change from base Build 3322086153:	4.6%
Covered Lines:	8732
Relevant Lines:	9724

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💛 - Coveralls