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
https://github.com/sqlparser-rs/sqlparser-rs/blob/ca15a4edbf1de4c00304b12e44d3adf9ac729406/src/ast/value.rs#L53

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%

---

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

---

💛 - Coveralls

[alamb]

Thanks @togami2864

[coveralls]

Pull Request Test Coverage Report for Build 2487582432

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 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%

---

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

---

💛 - Coveralls