[feature](nereids) implement uncheckedCast method in VarcharLiteral

[Kikyou1997]

Proposed changes

Issue Number: noissue

Problem summary

Implement uncheckedCast on varchar Literal for a temp way to let TimestampArithmetic work.
We should remove these code and do implicit cast in TypeCoercion rule in future.

Checklist(Required)

1. Does it affect the original behavior:
   • Yes
   • No
   • I don't know
2. Has unit tests been added:
   • Yes
   • No
   • No Need
3. Has document been added or modified:
   • Yes
   • No
   • No Need
4. Does it need to update dependencies:
   • Yes
   • No
5. Are there any changes that cannot be rolled back:
   • Yes (If Yes, please explain WHY)
   • No

Further comments

If this is a relatively large or complex change, kick off the discussion at dev@doris.apache.org by explaining why you chose the solution you did and what alternatives you considered, etc...

[Kikyou1997]

@morrySnow

[morrySnow]

please add regression test

[Kikyou1997]

please add regression test

added unit test