dump-signedexchange should decode mice payload

[nyaxt]

dump-signedexchange tool currently output raw MICE payload to stdout.
We should provide an option to decode+check integrity of MICE payload, and that should be the default behavior.

@hajimehoshi are you interested in owning this?

[irori]

Actually dump-signedexchange decodes MICE if -verify is given. It would be useful if MICE is decoded by default, though.