feat: support nydusify for build and push

[chlins]

This pull request introduces an experimental feature to "nydusify" model artifacts in both the build and push processes. The changes include updates to the configurations, command-line flags, and the implementation of the Nydusify function.

New Feature: Nydusify Model Artifacts

• Added a new Nydusify flag to the build and push configurations in pkg/config/build.go and pkg/config/push.go respectively. This flag is used to determine whether to nydusify the model artifact. [1] [2]
• Updated the init functions in cmd/build.go and cmd/push.go to include the new Nydusify flag and mark it as hidden. [1] [2]
• Implemented the Nydusify function in pkg/backend/nydusify.go, which converts a given image to a Nydus image using the nydusify command.

Build and Push Process Updates

• Modified the Build and Push methods in pkg/backend/build.go and pkg/backend/push.go to call the Nydusify function if the Nydusify flag is set. [1] [2]
• Added validation to ensure that the Nydusify flag can only be used when the OutputRemote flag is also set in the build configuration.

[aftersnow]

lgtm

[BraveY]

The invocation of nydusify depends on the image that has already been pushed to the image registry using modctl push. Therefore, calling nydusify directly without executing modctl push first is meaningless.