stratis-min: Detect bad format unlock method in parser and not at runtime (also for Uuid)

[mulkieran]

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In theory, we can do this with value_parser() method[1]. In practice, it is very hard to figure out how to use that method with the implementation of UnlockMethod, although in theory there is a macro value_parser which should work. All this is only minimally documented.

There is more or less the same problem for Uuid.

[1] https://docs.rs/clap/latest/clap/struct.Arg.html#method.value_parser