[Proposal] 70 - Router API

[Keith W]

Hey all,

https://github.com/kroxylicious/design/pull/70

Today in Kroxylicious each virtual cluster proxies exactly one Kafka cluster — the filter API can't change this.  This restriction blocks several use-cases such as:

- Union clusters: present multiple physical clusters as one — topics partitioned by prefix, brokers merged
- Principal-aware routing: use SASL identity to route users/tenants to different backends

- ....

Common thread: clients see one cluster; the proxy decides where things go

This proposal describes a Route API which is a general abstraction allowing the proxy to route to one or more clusters.