This PR:

• allows to configure gundeck to write to an additional redis (cluster or master mode); optionally. Reads will happen from the main redis.
• adds a test for the redis-migration scenario: configure to write to two redises (old and new); insert some data; configure only the new redis as default redis, read, ensure data is present.
• adds redis-cluster in docker-compose for local integration tests (a little more CPU/RAM needed)
• adds redis-cluster wrapper helm chart for CI/kubernetes-based integration tests (and more generally also for on-premise deployments). redis-cluster requires dynamic persistent volume provisioning; which was introduced in our kube-ci cluster with https://github.com/zinfra/cailleach/pull/1037

Drive-by refactoring change (not strictly needed on this PR):

• Move cannon's internal API type to wire-api

Related to https://github.com/zinfra/cailleach/pull/1021

This is needed to perform a smooth no-downtime migration from one redis to another as detailed in https://github.com/zinfra/cailleach/blob/master/targets/wire/notes-redis-migration.md#how-to-migrate-traffic-from-redis-in-ec2classic-to-redis-in-eks

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.
• 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