Add KEDA support for seldon-core

[anggao]

According to discussion in #2485
KEDA could be a good addon for seldon-core, and this ticket is used to track the work for it.

[anggao]

@cliveseldon FYI and I will track the work with this issue

[ukclivecox]

@anggao Can you discuss the scenarios you plan to use KEDA in w.r.t to the SeldonDeployment rpc model
I'd be interested to understand the metrics you plan to use to scale.

[anggao]

@cliveseldon we plan to use KEDA to further automate our deployment process. e.g. our internal tool can provide estimated cpu/memory usage, and throughput a model instance can handle. With KEDA it will help add the missing HPA part, we'd like to scale the model based on rps to the model, also in the future scale based on queue size would be of interest to offline training.