Add kuma support for progressive traffic shifting canaries

[johnharris85]

This PR adds support for Kuma service mesh to Flagger.

• All unit tests pass
• All e2e tests pass
• Docs added

Currently only canaries are supported, but I'm planning to add A/B testing in another PR when I get around to it.

[codecov-commenter]

Codecov Report

Merging #1085 (d203869) into main (ebc26e9) will increase coverage by 0.12%.
The diff coverage is 62.96%.

@@            Coverage Diff             @@
##             main    #1085      +/-   ##
==========================================
+ Coverage   57.23%   57.36%   +0.12%     
==========================================
  Files          76       78       +2     
  Lines        6162     6297     +135     
==========================================
+ Hits         3527     3612      +85     
- Misses       2105     2148      +43     
- Partials      530      537       +7     

Impacted Files	Coverage Δ
pkg/metrics/observers/factory.go	0.00% <0.00%> (ø)
pkg/router/factory.go	0.00% <0.00%> (ø)
pkg/metrics/observers/kuma.go	46.66% <46.66%> (ø)
pkg/router/kuma.go	70.27% <70.27%> (ø)

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[stefanprodan]

This looks great, thank you @johnharris85 🏅

[stefanprodan]

I don't think duplication is possible since the metric refers to a service by name and namespace which are unique per cluster. Can you add the same service to two Kuma meshes?

[stefanprodan]

LGTM

Thanks @johnharris85 🥇

[johnharris85]

Awesome, thanks for the review! I almost have blue-green finished too so if we can get this merged I'll throw that up later this week :)