core: CommandLine argument fails to fully capture strings with spaces

This is a spinout of a lingering issue detected when testing !2283 (merged).

The problem is that if a string with spaces is passed to a command line string variable, only the first substring before the whitespace will be saved in the string variable.

From Peter's suggestion here:

• need to change ATTRIBUTE_VALUE_IMPLEMENT_WITH_NAME. Peter states: "Not obvious to me if this will create problems elsewhere."

• we should add some test cases with spaces to `test-string-value-formatting.cc