Re organize the API package

[jian-he]

Right now the user facing APIs are scattered in different packages, we should re-organize them into a central api package.
A proposal looks like below:

1. types.go
2. [new file] interface.go: the ControllerInterface for custom operators to override
3. [new file] controller.go: the JobController struct which contains the ReconcileJobs entry method.
4. [new file] constants.go: the constant such as the label keys: job-name, replica-index, etc.

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[jian-he]

@richardsliu @johnugeorge @terrytangyuan opinions ?

[richardsliu]

Makes sense to me.

[terrytangyuan]

+1