Implement matchkeys interpolation function

[alvelcom]

This is a followup for #12389. The PR implements matchkeys function:

matchkeys(values, keys, searchset) - For two lists values and keys of equal length, returns all elements from values where the corresponding element from keys exists in the searchset list. For example

matchkeys(aws_instance.example.*.id, 
          aws_instance.example.*.availability_zone,
          list("us-west-2a"))

will return a list of the instance IDs of the aws_instance.example instances in "us-west-2a". No match will result in empty list. Items of keys are processed sequentially, so the order of returned values is preserved.

[ghost]

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