[Feature] Introducing Application level rollout and traffic management

[resouer]

For now we have implemented Component level rollout and traffic management.

But after we implemented #298 in server side, we will have the concept of Application and be able to do application level rollout and traffic mgmt - this case is also common in many app platform.

• Original design: [Proposal] - introduction of Deployment oam-dev/spec#342 (comment)
• A detailed design and workflow: [Proposal] - introduction of Deployment oam-dev/spec#342 (comment)

[wonderflow]

This is the application deployment we are working on: https://github.com/oam-dev/kubevela/blob/master/design/vela-core/rollout-design.md

[wonderflow]

We have implemented the Rollout https://kubevela.io/docs/rollout/rollout Traffic Management will be supported in the next release. I'll close this one and write a new issue #1357 .