sql: define default types for built-in functions

After this patch, all functions that take arguments of one of two or
more types have defined a default type, which is used when the argument
type cannot be determined.

Closes #6483

@TarantoolBot document
Title: Default types of SQL built-in functions

In case a function takes an argument of one of two or more types, this
function has defined the default type for this argument. This type is
used when the type of the argument cannot be determined, for example, in
the case of a bound variable.

Rules for determining default types as follows:
1) When there is only one possible type, it is default.
2) When possible types are INTEGER, DOUBLE or DECIMAL, DECIMAL is
   default.
3) When possible types are STRING or VARBINARY, STRING is default.
4) When possible data types are any other scalar data type, SCALAR is
   default.
5) Otherwise, there is no default type.