Implement KFP Template Registry API

[ckadner]

Description:

Implement the new "Generic Protocol for Kubeflow Pipeline Template Registry" API in MLX.

At high level, the protocol will define the generic APIs between KFP SDK client and a template registry server (MLX) to

• upload and download template (as YAML file), and
• organize templates as versioned objects
  The KFP SDK will implement (mid June 22) a registry client class that can work with any registry server that implements the APIs defined in the generic protocol.

References:

kubeflow/pipelines#7382

[ckadner]

@ptitzler