feat: add api for pod args

[hanxiao]

very first version of the pod api export, available at /data/api/pod (logserver.default.yml)

It returns {'pod': d, 'version': jina_version, 'usage': pod_cli_usage}, where d is the 'help', 'choices', 'default' and 'type' of each arg.

Limits:

• enum is stringify to int and becomes 0,1,2... semantics are lost
• default is only for reference, as some random default args will give random value everytime you call it.

[jina-bot]

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