chore(*) separate proxy template resolvers

[jpeach]

Summary

Breaking the dependency between the simple proxy template resolver and
the default fallback lets us specify the fallback as a default proxy
template resolver. This in turn, makes it possible for plugins to wrap
the default resolver so that this can be extended.

Moving the default ProxyTemplate resources out of pkg/xds/template
breaks a circular dependency between this package and xds/generator.

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 #2394 (3f90c52) into master (a7c748b) will increase coverage by 0.18%.
The diff coverage is 90.62%.

@@            Coverage Diff             @@
##           master    #2394      +/-   ##
==========================================
+ Coverage   52.35%   52.53%   +0.18%     
==========================================
  Files         875      877       +2     
  Lines       47816    47854      +38     
==========================================
+ Hits        25033    25142     +109     
+ Misses      20747    20664      -83     
- Partials     2036     2048      +12     

Impacted Files	Coverage Δ
pkg/xds/generator/proxy_template.go	85.71% <60.00%> (ø)
pkg/xds/template/proxy_template_resolver.go	62.50% <88.88%> (+21.32%)	⬆️
pkg/xds/server/v3/components.go	83.07% <100.00%> (+2.71%)	⬆️
pkg/core/resources/manager/cache.go	79.22% <0.00%> (-2.60%)	⬇️
pkg/mads/v1/client/client.go	41.25% <0.00%> (-2.50%)	⬇️
api/observability/v1/mads.pb.go	34.53% <0.00%> (-1.04%)	⬇️
api/internal/util/proto/proto.go	70.00% <0.00%> (ø)
api/internal/util/proto/types.go	100.00% <0.00%> (ø)
api/mesh/v1alpha1/dataplane_helpers.go	80.16% <0.00%> (+1.68%)	⬆️
pkg/kds/client/sink.go	52.27% <0.00%> (+4.54%)	⬆️
... and 6 more

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[lobkovilya]

But if the plugin overrides only DefaultTemplateResolver, does it mean we can't get rid of the calling SimpleProxyTemplateResolver? Probably it's better to define DefaultTemplateResolver like:

var DefaultTemplateResolver template.ProxyTemplateResolver = template.SequentialResolver(
 	&template.SimpleProxyTemplateResolver{
 		ReadOnlyResourceManager: rt.ReadOnlyResourceManager(),
 	},
 	&template.StaticProxyTemplateResolver{
 		Template: &mesh_proto.ProxyTemplate{
 			Conf: &mesh_proto.ProxyTemplate_Conf{
 				Imports: []string{mesh_core.ProfileDefaultProxy},
 			},
 		},
 	},
 )

and DefaultReconciller:

func DefaultReconciler(rt core_runtime.Runtime, xdsContext v3.XdsContext) xds_sync.SnapshotReconciler {
	return &reconciler{
		&templateSnapshotGenerator{
			ResourceSetHooks: rt.XDSHooks().ResourceSetHooks(),
			ProxyTemplateResolver: template.DefaultTemplateResolver,
		},
		&simpleSnapshotCacher{xdsContext.Hasher(), xdsContext.Cache()},
	}
}

WDYT?

[jpeach]

But if the plugin overrides only DefaultTemplateResolver, does it mean we can't get rid of the calling SimpleProxyTemplateResolver?
...
WDYT?

SimpleProxyTemplateResolver resolves the user-specified template, but when the user hasn't made a specific request, you want to be able to fall back to some other policy. This means that a plugin needs to be able to insert a resolver between the SimpleProxyTemplateResolver and whatever resolvers the xDS server falls back on. If both kinds of policy are combined in the DefaultTemplateResolver variable, there's no way for a plugin to insert a resolver into the correct place in the chain.