Add indexed attachment groups signature #234

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	Sha	Message	Pass or fail reason
🟢	c41eddf	add intermediate sig type	`Valid sign-off found`
🟢	c3e8dae	add reusable decode function for encodedsignature	`Valid sign-off found`
🟢	a2accbf	use signature decoder where appropriate	`Valid sign-off found`
🟢	39bfc71	add mandatory codes	`Valid sign-off found`
🟢	f66ca6e	add explicit checks on the sizes of the signature	`Valid sign-off found`
🟢	f678bc1	Fix color formatting for signature	`Valid sign-off found`