Bumps the version of the generated encrypted blob from `2` to `3`.

To allow decryption of the older version `2` encrypted blobs with the
same binary, we import both versions of the `hpke` crate.
This is behind the optional `blob_legacy_version` cargo feature,
enabled by default. Decryption of both blob versions will happen
transparently by leveraging the prepended version byte.

We're keeping most of this as copy-paste, but may eventually
encapsulate it properly if the upgrade path continues to resemble
this one, i.e., same interface but different types.