Add DomainMapping CRD and type

[julz]

First part of #9713.

Adds a DomainMapping CRD in config/domain-mapping and a new corresponding type in a resurrected apis/serving/v1alpha1 package, and runs code-gen.

/assign @mattmoor @dprotaso

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@89fc5bd). Click here to learn what that means.
The diff coverage is 41.17%.

@@            Coverage Diff            @@
##             master    #9714   +/-   ##
=========================================
  Coverage          ?   87.98%           
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  Files             ?      179           
  Lines             ?     8455           
  Branches          ?        0           
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  Hits              ?     7439           
  Misses            ?      772           
  Partials          ?      244           

Impacted Files	Coverage Δ
cmd/autoscaler/main.go	13.26% <0.00%> (ø)
...g/apis/serving/v1alpha1/domainmapping_lifecycle.go	100.00% <100.00%> (ø)
pkg/apis/serving/v1alpha1/domainmapping_types.go	100.00% <100.00%> (ø)
pkg/apis/serving/v1alpha1/register.go	100.00% <100.00%> (ø)
pkg/autoscaler/bucket/bucket.go	100.00% <100.00%> (ø)
pkg/reconciler/autoscaling/kpa/scaler.go	88.57% <100.00%> (ø)

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[mattmoor]

This generally LGTM. You plan to add Validate/Default (and wire into webhook) in a follow-on?

[julz]

Yep, added that to the plan of attack in the tracking issue @mattmoor, just trying to keep each PR reasonably small

[mattmoor]

/lgtm
/approve

🎉

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