Add constants for supported duck labels and annotations

[antoineco]

Problem
We don't have any constant set for common labels and annotations such as

• duck.knative.dev/addressable
• duck.knative.dev/source
• ...

This can lead to repetition in the code base, especially in projects that use Knative as a dependency.

Persona:
Contributors

Exit Criteria
Supported labels and annotations are defined as public constants inside pkg/apis/duck

Example: https://github.com/knative/serving/blob/3b5be63/pkg/apis/serving/register.go

Time Estimate (optional):
How many developer-days do you think this may take to resolve?
1/2