This caching is enabled by wrapping the `secrets.Manager` returned by
`DefaultSecretsProvider.OfType` in an outer `secrets.Manager` that
cooperates with `stack.{Serialize,Deserialize}PropertyValue`. Ciphertext
is cached on a per-secret-instance basis (i.e. not a per-plaintext-value
basis). Cached ciphertext is only reused if the plaintext for the secret
value has not changed. Entries are inserted into the cache upon both
encryption and decryption so that values that originated from ciphertext
and that have not changed can aoid re-encryption.
Contributes to #3178.
When constructing a Deployment (which is a plaintext representation of
a Snapshot), ensure that we encrypt secret values. To do so, we
introduce a new type `secrets.Manager` which is able to encrypt and
decrypt values. In addition, it is able to reflect information about
itself that can be stored in the deployment such that we can
deserialize the deployment into a snapshot (decrypting the values in
the process) without external knowledge about how it was encrypted.
The ability to do this is import for allowing stack references to
work, since two stacks may not use the same manager (or they will use
the same type of manager, but have different state).
The state value is stored in plaintext in the deployment, so it **must
not** contain sensitive data.
A sample manager, which just base64 encodes and decodes strings is
provided, as it useful for testing. We will allow it to be varried
soon.
