Introduce Signature::String and return error if input of strpos is integer

[jayzhan211]

Which issue does this PR close?

Rationale for this change

I think #12712 introduce a regression on integer type, this PR introduce another signature for string, and return error for integer input on strpos function, which has the same result as Postgres

postgres=# select position(1 in 1);
ERROR:  function pg_catalog.position(integer, integer) does not exist
LINE 1: select position(1 in 1);
               ^
HINT:  No function matches the given name and argument types. You might need to add explicit type casts.

What changes are included in this PR?

Are these changes tested?

Are there any user-facing changes?

Others

Not sure is this generalize to all the string related function. Are they all expect converting null to String? Handle dictionary?