Implement AppMesh v1beta2 router

[stefanprodan]

This PR adds support for AppMesh v1beta2. Since v1beta2 is not backwards compatible, users must set the provider to appmesh:v1beta2 on new cluster in order to use Flagger with the new AppMesh manager.

Changes:

• add AppMesh v1beta2 router (provider appmesh:v1beta2)
• maintain compatibility with AppMesh v1beta1 (provider appmesh)
• register VirtualNodes before primary deployment initialisation
• add provider list to API
• refactor observer factory
• add Envoy metric templates to crossover docs
• fix service annotation diff

TODO (followup PRs):

• add v1beta2 VirtualNodes to loadtester helm chart
• rewrite the Flagger AppMesh Gateway for v1beta2
• create docs for AppMesh v1beta2 setup on EKS
• rewrite AppMesh tutorial when v1beta2 is released by AWS

Ref: #577

[codecov-io]

Codecov Report

Merging #584 into master will increase coverage by 2.36%.
The diff coverage is 75.12%.

@@            Coverage Diff             @@
##           master     #584      +/-   ##
==========================================
+ Coverage   52.72%   55.08%   +2.36%     
==========================================
  Files          63       62       -1     
  Lines        4922     5221     +299     
==========================================
+ Hits         2595     2876     +281     
- Misses       1924     1928       +4     
- Partials      403      417      +14     

Impacted Files	Coverage Δ
pkg/controller/finalizer.go	39.75% <0.00%> (ø)
pkg/metrics/observers/factory.go	0.00% <0.00%> (ø)
pkg/router/factory.go	0.00% <0.00%> (ø)
pkg/controller/scheduler.go	46.88% <22.22%> (-0.33%)	⬇️
pkg/router/kubernetes_default.go	64.82% <50.00%> (-0.91%)	⬇️
pkg/router/appmesh_v1beta2.go	86.88% <86.88%> (ø)

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