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.
