Add HPKE key generation tool

[divergentdave]

This adds a command-line tool to generate HPKE keypairs, closing #801. Sample output:

# HPKE configuration, Janus format
id: 255
kem_id: X25519HkdfSha256
kdf_id: HkdfSha256
aead_id: Aes128Gcm
public_key: vaiPHYKo6c_HZvqjMgfcFLgWmvo45nbJ_5G4ShFBQnc
---
# HPKE private key, in base64url
EC9SQqZky66zQ5dl1aWu-5xZ34MVfB2RdoSCFGkHlGo
---
# HPKE keypair, Janus format
config:
  id: 255
  kem_id: X25519HkdfSha256
  kdf_id: HkdfSha256
  aead_id: Aes128Gcm
  public_key: vaiPHYKo6c_HZvqjMgfcFLgWmvo45nbJ_5G4ShFBQnc
private_key: EC9SQqZky66zQ5dl1aWu-5xZ34MVfB2RdoSCFGkHlGo
---
# HPKE configuration, DAP encoded, in base64url
_wAgAAEAAQAgvaiPHYKo6c_HZvqjMgfcFLgWmvo45nbJ_5G4ShFBQnc

Fun fact: since the various ID fields and the public key's length prefix are nine bytes all together, a multiple of three, the base64url-encoded public key appears verbatim in the base64url-encoded HpkeConfig message.