Simplify DIDComm unpack(), use `Key` instead of `String` #7855

Job	Run time
workflow-setup	20s
verify-code-formatting	25s
clippy	3m 34s
workflow-setup-check	1s
build-docker-vdrproxy	7s
build-docker-aath-backchannel	0s
test-integration-aries-vcx (askar_wallet,anoncreds)	8m 51s
test-integration-did-crate	2m 7s
test-unit-workspace	1m 48s
publish-docker-aath-backchannel	0s
test-integration-aries-vcx-vdrproxy	14m 35s
publish-docker-vdrproxy	1s
make-release	0s
	31m 49s

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