[WIP] Implement TrainerClient Backends & Local Process

[szaher]

What this PR does / why we need it: This PR introduces

• Training backends (can be renamed to workers if backend isn't suitable)
• Basic Implementation of training backends registry (potentially extending to other backends like Docker, Podman, aws, ...etc)
• Rework TrainerClient to initialize and calls selected backend
• Uses k8s as default backend for TrainerClient
• Implements local process as new training backend.
• Introduces LocalTrainingRuntime to enable users to define their own run times in memory or use available ones for local training

Which issue(s) this PR fixes (optional, in Fixes #<issue number>, #<issue number>, ... format, will close the issue(s) when PR gets merged):

Fixes #

Checklist:

• Docs included if any changes are user facing

[google-oss-prow]

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