Support encrypting plaintexts longer than the RSA limit

[kgpayne]

RSA has a maximum plaintext size in bytes, as documented here. To overcome this maximum, two approaches present themselves:

1. Chunk plaintexts according to the supported maximums, and support the concatenation of plaintexts to reconstitute long secrets.
2. Use envelope encryption, whereby RSA is used to encrypt a symmetric encryption key corresponding to an encryption scheme without byte limits (e.g. AES)

From a brief discussion F2F it was decided that option 1 would be the simplest for now.