add interval

[daviszhen]

What type of PR is this?

• API-change
• BUG
• Improvement
• Documentation
• Feature
• Test and CI
• Code Refactoring

Which issue(s) this PR fixes:

issue #24486

What this PR does / why we need it:

增加interval函数

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[XuPeng-SH]

I found two blocking issues.

1. The binder still steals some legal INTERVAL() function calls away from the new builtin path. In this patch base_binder.go rewrites interval(...) to T_interval whenever isIntervalExprSyntax(args) sees exactly two arguments and the second one is a string literal matching an interval unit. But the builtin itself is registered to accept string arguments, so calls like interval(5, 'day') never reach the new numeric INTERVAL() implementation even though they are valid under the registered signature. This keeps the old interval-expression path for a subset of function-style calls instead of fully disambiguating the new builtin.

2. The string conversion semantics are not MySQL-compatible yet. makeIntervalParam() uses strconv.ParseFloat, so inputs such as '', ' ', '1abc', 'abc', and '10.9abc' raise hard errors. The new BVT file even documents the intended behavior as “return values with warnings, not fail”, but the checked-in .result records errors instead. That means the implementation and coverage are both locking in behavior that diverges from MySQL for the new function.