[mutation] Cache tester in AssignMetadata like we do in Assign mutators

[willbeason]

Note: depends on #1441

Per findings in #1441, we can greatly improve the speed of AssignMetadata.Mutate by caching the tester instead of recreating it every time we call Mutate.

This reduces the runtime when no mutation is made by 97%, and when mutations are made by 30%!

$ for i in {1..20}; do echo $i; go test github.com/open-policy-agent/gatekeeper/pkg/mutation/mutators --bench=BenchmarkAssignMetadata >> before.out; done
$ git checkout benchmark-metadata-2
$ for i in {1..20}; do echo $i; go test github.com/open-policy-agent/gatekeeper/pkg/mutation/mutators --bench=BenchmarkAssignMetadata >> after.out; done
$ benchstat before.out after.out 

name                        old time/op  new time/op  delta
AssignMetadataMutator_Always-12    6.64µs ± 2%  4.70µs ± 1%  -29.22%  (p=0.000 n=19+19)
AssignMetadataMutator_Never-12     1.84µs ± 6%  0.05µs ± 0%  -97.04%  (p=0.000 n=19+18)

I'll send a separate PR which removes the unnecessary json unmarshalling in the case where we perform the mutation, once/if #1439 is merged.

[codecov-commenter]

Codecov Report

Merging #1442 (458774d) into master (152e102) will increase coverage by 0.25%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #1442      +/-   ##
==========================================
+ Coverage   50.35%   50.60%   +0.25%     
==========================================
  Files          77       77              
  Lines        5108     5110       +2     
==========================================
+ Hits         2572     2586      +14     
+ Misses       2185     2174      -11     
+ Partials      351      350       -1     

Flag	Coverage Δ
unittests	50.60% <50.00%> (+0.25%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pkg/mutation/mutators/assignmeta_mutator.go	47.82% <50.00%> (+4.54%)	⬆️
...onstrainttemplate/constrainttemplate_controller.go	54.69% <0.00%> (+1.61%)	⬆️
pkg/readiness/object_tracker.go	82.60% <0.00%> (+1.63%)	⬆️
pkg/controller/assign/assign_controller.go	51.72% <0.00%> (+1.72%)	⬆️

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[maxsmythe]

LGTM

@shomron @sozercan LGTY?