Move ast::ColumnOption::Default variant to ColumnDef

[panarch]

Each ColumnDef can have at most one ColumnOption::Default but current ast structure allows ColumnDef to have multiple Defaults.
Resolve this by adding default field to ColumnDef and remove ColumnOption::Default.

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It can also be applied to ColumnOption::Unique which is the last enum variant of ColumnOption.

[coveralls]

Pull Request Test Coverage Report for Build 3662653145

• 67 of 67 (100.0%) changed or added relevant lines in 11 files are covered.
• 14 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.003%) to 98.559%

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Files with Coverage Reduction	New Missed Lines	%
core/src/executor/evaluate/mod.rs	7	97.48%
core/src/result.rs	7	69.77%

Totals
Change from base Build 3630622723:	0.003%
Covered Lines:	37336
Relevant Lines:	37882

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💛 - Coveralls

[ever0de]

Looks good