Better optional parameter support in testing

[not-napoleon]

The testFunctionInfo method in the ES|QL parameterized testing gets confused when there are optional parameters in the function signature. See for example DateFormat, which has two forms: DATE_FORMAT(format_string, date) and DATE_FORMAT(date). The first positional argument can be a string or a date. In #120000, I worked around it by adding datetime to the acceptable types for the first parameter, but we could be more clever in testFunctionInfo. We can detect if the parameter is optional, and adjust behavior accordingly.

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)