cmArgumentParser: Add structure to capture operational results

Create a way for the parser to record errors or incidental information about the argument parsing operation that clients can inspect afterward. Offer clients a way to hold the structure as part of their arguments structure.

Since !7459 (merged) we know during parsing whether or not it is an error for a keyword to be missing a value. Record such errors in the parse results structure. Offer clients a helper method to report them. This provides clients with an alternative to manually checking keywordsMissingValue and generating their own error message.