Move ContainerSource to v1beta1

[nachocano]

Problem
We need to promote ContainerSource to v1beta1.
Some tasks this will entail, and can probably be done in different PRs.

• v1beta1 types with UTs
• add new version to CRD
• defaulting and conversion webhook
• Update docs and examples (most in knative/docs)
• Migrate reconciler to v1beta1 with UTs
• Update e2e tests to use v1beta1

Persona:
Developer

Exit Criteria
ContainerSource in v1beta1

Additional context (optional)
Some example PRs
types: #2407
defaulting: #2645
conversion: #2611, #2632
reconciler: #3075

[nachocano]

fyi @bryeung

[nachocano]

Synced offline with @bharattkukreja.
Decided to have e2e tests for v1alpha2 and v1beta1. So we should follow what SinkBinding is doing in their tests.
This should be done in multiple PRs as mentioned in the description

[matzew]

@n3wscott do we have similar issues for PingSource and SinkBi ding?

[nachocano]

We should move flip the storage bit in 0.17. We should do so for 0.18.
And we should add a migration tool.
@bharattkukreja can you create an issue similar to #3615 to track that once this one is done?