feat(kuma-cp) initial gateway TrafficRoute support

[jpeach]

Summary

Add support for binding TrafficRoute resources to Gateways. This is
based on outbound proxy generator code, but there are still a number of
missing features and outstanding TODO items.

It is possible to configure simple HTTP Gateway use cases and forward
traffic to Kuma services as long as the mesh is not configures with mTLS.
A Gateway can have more than one TrafficRoute resource bound, and this
works correctly by merging the routes in match order.

Full changelog

N/A

Issues resolved

N/A

Documentation

N/A

Testing

• Unit tests
• E2E tests
• Manual testing on Universal
• Manual testing on Kubernetes

Backwards compatibility

• Add backport-to-stable label if the code is backwards compatible. Otherwise, list breaking changes.

[codecov-commenter]

Codecov Report

Merging #2547 (e0fc011) into master (7793853) will increase coverage by 0.19%.
The diff coverage is 79.66%.

@@            Coverage Diff             @@
##           master    #2547      +/-   ##
==========================================
+ Coverage   52.25%   52.45%   +0.19%     
==========================================
  Files         865      872       +7     
  Lines       49300    49662     +362     
==========================================
+ Hits        25761    26049     +288     
- Misses      21453    21510      +57     
- Partials     2086     2103      +17     

Impacted Files	Coverage Δ
pkg/xds/envoy/routes/configurers.go	90.90% <0.00%> (-9.10%)	⬇️
pkg/plugins/runtime/gateway/builder.go	72.72% <63.63%> (-9.10%)	⬇️
.../plugins/runtime/gateway/virtual_host_generator.go	71.42% <71.42%> (ø)
pkg/plugins/runtime/gateway/match/gateway.go	75.00% <75.00%> (ø)
pkg/plugins/runtime/gateway/merge/traffic_route.go	76.19% <76.19%> (ø)
pkg/plugins/runtime/gateway/generator.go	76.59% <76.59%> (ø)
...plugins/runtime/gateway/traffic_route_generator.go	79.83% <79.83%> (ø)
pkg/plugins/runtime/gateway/match/util.go	85.71% <85.71%> (ø)
pkg/plugins/runtime/gateway/match/routes.go	86.66% <86.66%> (ø)
...s/runtime/gateway/route_configuration_generator.go	86.66% <86.66%> (ø)
... and 21 more

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[jpeach]

@lobkovilya sorry about the force push, my working copy was a bit behind my fork :-(

[jakubdyszkiewicz]

There are lots of todos but as a concept, we are heading in a good direction.

[jpeach]

I've addresses all the review feedback so far. Thanks!