Make ContainerSource v1beta1 the stored version and add a migration tool

[bharattkukreja]

Problem
In 0.17, we promoted ContainerSource to v1beta1.
This is to track that we should make v1beta1 the storage version for 0.18, and we should add a migration tool for it.
Similar to: https://github.com/knative/eventing/blob/master/config/pre-install/v0.16.0/storage-version-migration.yaml

Persona:
Which persona is this feature for?
Developer

Exit Criteria
ContainerSource in v1beta1 and able to migrate

Additional context (optional)
#3426

[grantr]

Blocked until 0.17 release.

[nachocano]

/assign @nlopezgi