[V3] Add the new schema v3alpha1.

This schema contains the main fields to support the new skaffold render and deployment, powered by kpt. Some config names may change based on the the user survey results.

For reviewers:
 - `Render`: Top tier section. It contains the basic render functions `generate`, `validate` and `trasform`. It does not include the helmcharts (generate) and kpt package magement yet, which will be added on following CLs.
 - `Deploy`: Simplified. The deployers (helm, kubectl, kpt, kustomize) are removed. Some helm and kubectl features, and flags are not covered in the `deploy` section yet. Will be added in following CLs.
 -  v2 and v3 versions are not compatible. So the `upgrade` functions do not need to change. But we will add more checks on other codes to guarantee the v2 and v3 are not mixed (especially in multi-config mode).
 -  Generated doc via "go run hack/schemas/main.go".

Author: yuwenma