Support for IF NOT EXISTS in ALTER TABLE ADD COLUMN

[AugustoFKL]

• Add support for IF NOT EXISTS on PostgreSQL (1), BigQuery (2), and Generic.
• Add COLUMN displaying properly, to match the original query.

[1] : https://www.postgresql.org/docs/current/sql-altertable.html
[2] : https://cloud.google.com/bigquery/docs/reference/standard-sql/data-definition-language#alter_table_rename_column_statement

[coveralls]

Pull Request Test Coverage Report for Build 3455920090

• 36 of 39 (92.31%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 86.29%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/parser.rs	4	5	80.0%
src/ast/ddl.rs	10	12	83.33%

Totals
Change from base Build 3448124733:	0.01%
Covered Lines:	11839
Relevant Lines:	13720

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💛 - Coveralls

[AugustoFKL]

@alamb don't want to hurry or anything, but are you waiting for anything to review this?

I have quite a few PR's in row hehe

[alamb]

@alamb don't want to hurry or anything, but are you waiting for anything to review this?

No I am not waiting on anything. I will try and find time to work through the sqlparser-rs review backlog. Thanks for bringing it to my attention

[alamb]

Thank you @AugustoFKL