MLS key packages

[pcapriotti]

Implementation of MLS key package API. This PR introduces 3 new endpoints for managing key packages, and corresponding schema changes:

• POST /mls/key-packages/self/:client
• GET /mls/key-packages/self/:client/count
• POST /mls/key-packages/claim/:domain/:user

Key packages are claimed with a locking strategy similar to that for prekeys (in the absence of DynamoDB).

This PR also introduces some basic MLS types and corresponding deserialisation primitives (for its default TLS encoding).

Only local behaviour is implemented in this PR. Federation behaviour, as well as key package mapping (needed in order to be able to resolve key package refs to qualified client IDs), is left for future PRs.

Note: integration tests rely on the crypto-cli tool being in the path. The tool can be found here (note that it is a branch).

Tracked by https://wearezeta.atlassian.net/browse/FS-431.

TODO

• Install crypto-cli in the dev environment
• Install crypto-cli in CI

Checklist

• The PR Title explains the impact of the change.
• The PR description provides context as to why the change should occur and what the code contributes to that effect. This could also be a link to a JIRA ticket or a Github issue, if there is one.
• If HTTP endpoint paths have been added or renamed, the endpoint / config-flag checklist (see Wire-employee only backend wiki page) has been followed (cailleach PR: https://github.com/zinfra/cailleach/pull/897).
• If a cassandra schema migration has been added, I ran make git-add-cassandra-schema to update the cassandra schema documentation.
• changelog.d contains the following bits of information (details):
  • A file with the changelog entry in one or more suitable sub-sections. The sub-sections are marked by directories inside changelog.d.
  • If new config options introduced: added usage description under docs/reference/config-options.md
  • If a data migration (not schema migration) introduced: measures to be taken by on-premise instance operators.
  • If public end-points have been changed or added: does nginz need un upgrade?

[mdimjasevic]

This is great! I have some minor questions; they're inlined.