JMESPath type checker completeness

[0marperez]

The jmespath type checker is not complete

For example:

FUNCTIONS.put("sort", new FunctionDefinition(ARRAY, isArray));

The sort function type check is not looking at type of array when the jmespath specification says it should be an array of string or number only.

FUNCTIONS.put("sort_by", new FunctionDefinition(ARRAY, isArray, isType(RuntimeType.EXPRESSION)));

Same for the sort_by function, the expression should only result in a string or number.

I see TODO's in there but no issue created to keep track of it. It'd be good to have this completed as it could cause invalid models.