[CometFuzz] Add support for scalar arguments, with custom generator for valid values

[andygrove]

What is the problem the feature request solves?

Currently, CometFuzz only passes columns to functions. For functions such date_truc, it is more typical for users to pass a scalar string in for the format, and we currently do not fuzz test for that example. Also, using the same example, there are a small number of valid values for format, so we should have a way to provide a list of valid scalar inputs.

Another example would be rpad where it doesn't make sense to use Int.MaxValue for the length, We should be able to specify to use smaller values.

Describe the potential solution

Perhaps some way to provide a custom generator per function per input

Additional context

No response