embedded-services: Add support for notifications

[kurtjd]

We need to add a transport agnostic means of representing notification messages (that tasks can send asynchronously to relay services).

[kurtjd]

Will wait for #703 to be fully completed before tackling this.

[kurtjd]

Should be ready to make progress on this now after relay updates.

[kurtjd]

It seems I keep missing the mark with this one. Difficult to design when we don't have a strong use case or requirements yet. Will revisit when the need actually arises.