Simplify DIDComm unpack(), use `Key` instead of `String` #7850

Job	Run time
workflow-setup	31s
workflow-setup	31s
verify-code-formatting	38s
verify-code-formatting	33s
workspace_clippy (askar_wallet)	3m 57s
workspace_clippy (askar_wallet)	3m 57s
aries_vcx_clippy (anoncreds,askar_wallet)	2m 35s
aries_vcx_clippy (anoncreds,askar_wallet)	2m 35s
aries_vcx_clippy (vdr_proxy_ledger,askar_wallet)	2m 5s
aries_vcx_clippy (vdr_proxy_ledger,askar_wallet)	2m 5s
test-unit-workspace	2m 31s
test-unit-workspace	2m 31s
workflow-setup-check	1s
workflow-setup-check	1s
build-docker-vdrproxy	5s
build-docker-vdrproxy	5s
build-docker-aath-backchannel	0s
build-docker-aath-backchannel	0s
test-integration-aries-vcx (askar_wallet,anoncreds)	6m 41s
test-integration-aries-vcx (askar_wallet,anoncreds)	6m 41s
test-integration-did-crate	1m 54s
test-integration-did-crate	1m 54s
publish-docker-aath-backchannel	0s
publish-docker-aath-backchannel	0s
test-integration-aries-vcx-vdrproxy	11m 55s
test-integration-aries-vcx-vdrproxy	11m 55s
publish-docker-vdrproxy	2s
publish-docker-vdrproxy	2s
make-release	0s
make-release	0s
	1h 5m 45s

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