Nmarton/explicit disclosure api with create argument blob

[nmarton-da]

[nmarton-da]

This is a follow up PR on top of API changes needed for interface subscriptions here, which extracts and encapsulates the changes for enhancing explicit disclosure with opaque payload.

This PR is not to be merged, just serves as a forum to foster collaboration. Merging this needs to be orchestrated with the respective implementation, which should follow the rollout of both interface subscriptions and explicit disclosure.

[nmarton-da]

superseded by real implementation