fix: Support expr instead of string for argument to interval

[togami2864]

In Big Query and MySQL, the following query is valid:

SELECT DATE_ADD(DATE "2008-12-25", INTERVAL 5 DAY);

https://cloud.google.com/bigquery/docs/reference/standard-sql/date_functions#date_add
https://www.w3schools.com/mysql/func_mysql_date_add.asp

But now if the expression is number, an error occurred because the type of the value in interval is String

sqlparser-rs/src/ast/value.rs

Line 53 in ca15a4e

value: String,

This PR replaces String with Expr to run correctly in Interval

One concern thing is that should I validate when the other dialect like PostgresSQL, sqlite. I don't know if the query is valid except for Big query and MySQL.

[46bit]

Great to see this. There's related discussion in #298.

[alamb]

Looks good to me -- thank you @togami2864

[alamb]

@togami2864 -- there appear to be some CI failures

[togami2864]

@alamb I fixed all CI failures. Thanks.

[coveralls]

Pull Request Test Coverage Report for Build 2487582432

• 28 of 29 (96.55%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 89.822%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/ast/value.rs	1	2	50.0%

Totals
Change from base Build 2450394803:	0.03%
Covered Lines:	8737
Relevant Lines:	9727

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💛 - Coveralls

[alamb]

Thanks @togami2864