This PR integrates a post-quantum ratchet (SPQR) into libsignal, using an API that maintains its own internal chain and provides per-message keys. In doing so, it also aims to be fully backwards-compatible with current clients and stored session state.
## Backwards compatibility with current clients
Remote clients that connect to us or that we connect to may not have this integration. If they don't, their SignalMessage wire format should still deserialize, and in doing so we'll receive an empty pq_ratchet field. SQPR handles this internally, by downgrading the protocol version to "version 0" or "don't do anything". Note that should we eventually want to disallow this, we can do so via increasing the `min_version` field passed into the SQPR init functions to V1. This is also the method by which we would upgrade SQPR from v1 to a future v2, etc.
## Opt-in
The publicly facing API calls for this now expose an explicit opt-in via a passed-in `use_pq_ratchet` bool (and associated enums in language-specific APIs). If false, they default to SQPR `v0`, IE: none. If true, they try to set up SPQR on new sessions, but will downgrade if the remote party cannot or will not do the same.
...in favor of the more general functions using
UnidentifiedSenderMessageContent, which the iOS app is already using.
(sealedSenderDecrypt in particular was never updated to support
PQXDH.)
For the most part this should happen transparently without any
explicit adoption, like the previous change, but for Java code the
NoSessionException is now properly declared on SessionCipher.encrypt.
(This was always technically possible, but clients were expected to
have previously checked for session validity before using
SessionCipher; now that there's an expiration involved, that's not
strictly possible.)
Like ProtocolAddresses in 88a2d5c, these APIs will eventually only
support ACIs, so introducing strong types now helps move in that
direction. However, the existing APIs that produce strings have not
been removed yet.
