Add Redis backed Event Store

[srempfer]

• Mainly for testing if Travis CI supports Redis and what have to be done to get it working

[coveralls]

Coverage decreased (-0.7%) to 90.58% when pulling 5fc4af8 on srempfer:gh-684 into 53ce34a on codecentric:master.

[jonashackt]

We need to implement a GitHub Actions version of the Redis example also - for example see https://github.com/marketplace/actions/redis-server-in-github-actions

[SteKoe]

memo: addresses #684

[shubhamjambhe8]

I know this PR is in draft but tried this PR implementation in one of my project with Redis Sentinel configuration, here are my observations :-

1. when using one instance of admin server & N instance of client it works perfect
2. when used N instance of admin server behind a loadbalancer / proxy & N instance of client it fails (consistency between multiple admin server instance is not present) due to following reason :-
   lets say we have
   - 2 Admin (Admin A, Admin B) server apps behind LB
   - 4 Client (Client 1-4) apps registering themselves via LB endpoint
   i) Now assume if Client 1,3 registers themselves (with status UP) to Admin A app & client 2,4 registers themselves (with status UP) to Admin B,
   ii) during this scenario Admin A does status/info updates only for Client 1,3 it will save the updates in redis data store & its local snapshot repository (using default SnapshottingInstanceRepository)
   iii) similarly Admin B does status/info updates only for Client 2,4 it will save the updates in redis data store & its local snapshot repository (using default SnapshottingInstanceRepository)
   iv) Now since only local snapshot repository are updated the admin UI are not are in sync, for eg:- Admin B might show status of Client 1,3 as UNKNOWN / whatever was the last saved event present in redis for that instance

Possible Fix :- Use pub/sub model between Redis eventstores to publish any saved events by any Admin server instance so other admin instance knows about the change just like HazelcastEventStore

[SteKoe]

As this is slightly outdated and needs to be bumped to Spring Boot 3 as well, I will close this PR for now.