Introduce DomainMapping ConfigStore and allow configuring IngressClass

[julz]

Fifth part of #9713.

Respect the default ingressClass in network.yaml and allow overriding via annotation, analogously to Route.

/assign @mattmoor @dprotaso

[codecov]

Codecov Report

Merging #9796 into master will decrease coverage by 0.00%.
The diff coverage is 68.00%.

@@            Coverage Diff             @@
##           master    #9796      +/-   ##
==========================================
- Coverage   87.76%   87.76%   -0.01%     
==========================================
  Files         180      181       +1     
  Lines        8552     8574      +22     
==========================================
+ Hits         7506     7525      +19     
- Misses        797      801       +4     
+ Partials      249      248       -1     

Impacted Files	Coverage Δ
pkg/reconciler/domainmapping/controller.go	0.00% <0.00%> (ø)
pkg/reconciler/domainmapping/config/store.go	100.00% <100.00%> (ø)
pkg/reconciler/domainmapping/reconciler.go	85.36% <100.00%> (+0.36%)	⬆️
cmd/queue/main.go	0.51% <0.00%> (+0.51%)	⬆️
pkg/activator/net/revision_backends.go	91.40% <0.00%> (+0.90%)	⬆️
pkg/reconciler/revision/reconcile_resources.go	83.13% <0.00%> (+2.40%)	⬆️

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[julz]

/retest

[mattmoor]

/lgtm
/approve

Awesome 🤩

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: julz, mattmoor

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