Add support for generated columns skipping 'GENERATED ALWAYS' keywords

[takluyver]

SQLite, Mysql and DuckDB all let you skip the words GENERATED ALWAYS, and use AS to introduce a column generated from an expression. See the links in #1051 for details.

I've introduced an extra field in ColumnOption::Generated - generated_kw is true if the GENERATED keyword is present, and false if the SQL skips directly to AS. I'm not sure of the naming, but it's the best I've thought of so far. Currently, if generation_expr is None, generated_kw can only be true (and is assumed to be when recreating the SQL).

Perhaps a neater design would be to have different ColumnOption kinds for generated-from-expression vs. generated-from-sequence vs. whatever else. But that would require an API break again.

Closes #1050.

[coveralls]

Pull Request Test Coverage Report for Build 7052140096

• 26 of 32 (81.25%) changed or added relevant lines in 4 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.01%) to 87.695%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/parser/mod.rs	16	22	72.73%

Files with Coverage Reduction	New Missed Lines	%
src/ast/ddl.rs	1	83.78%

Totals
Change from base Build 7009122777:	-0.01%
Covered Lines:	18017
Relevant Lines:	20545

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💛 - Coveralls

[tobyhede]

Personal preference, but I would be inclined to expand the name to generated_keyword for clarity.

[takluyver]

No problem, I've done that renaming.

[tobyhede]

LGTM

[alamb]

Looks good to me -- thank you @takluyver and thank you @tobyhede for the review

[takluyver]

Thanks!