A new `Secret` property value is introduced, and plumbed across the
engine.
- When Unmarshalling properties /from/ RPC calls, we instruct the
marshaller to retain secrets, since we now understand them in the
rest of the engine.
- When Marshalling properties /to/ RPC calls, we use or tracked data
to understand if the other side of the connection can accept
secrets. If they can, we marshall them in a similar manner to assets
where we have a special object with a signiture specific for secrets
and an underlying value (which is the /plaintext/ value). In cases
where the other end of the connection does not understand secrets,
we just drop the metadata and marshal the underlying value as we
normally would.
- Any secrets that are passed across the engine events boundary are
presently passed as just `[secret]`.
- When persisting secret values as part of a deployment, we use a rich
object so that we can track the value is a secret, but right now the
underlying value is not actually encrypted.
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