Add v1alpha3 API types

[prksu]

We already cut a release for v1alpha2 API types in v0.3.x which compatible with capi v0.2.x. We need to add v1alpha3 types as the first step to make capdo also compatible with capi v0.3.x.

• Copy types from api/v1alpha2.
• Rewrite imports.
• Set the storage version to v1alpha3 types.
• Make sure the generated CRDs contain both v1alpha2 and v1alpha3.

For the detail about migrating v1alpha2 to v1alpha3 can be found here

/kind feature
/milestone v0.4
/priority important-soon

[prksu]

/assign