Add ConfigMap support to resolvers so they can support admin configuration

[ghost]

Feature request

Right now any resolver based on pkg/resolver/framework does not have an easy way to pull in admin configuration options. In any realistic production-ready resolver it's highly likely that some amount of configuration will be required:

• timeouts <- this is a big one
  • right now the core ResolutionRequest reconciler has a hard-coded global timeout of 60 seconds
  • independent of the 60 second global timeout, every resolver based on framework.Resolver interface has a hard-coded 30 second timeout
• an allow-list of repositories that a git resolver is able to fetch from
• the names of secrets to use for auth
• namespaces of tenants allowed to use the resolver
• the API endpoint to hit for a private git server
• ... or any of a thousand different parameters that a resolver developer might want to support

So the feature request is to add a new optional interface for Resolvers to implement. Something like:

// ResolverConfigMap is an optional interface for a resolver to implement that allows it
// to register the name of a configmap that holds its admin settings.
type ResolverConfigMap interface {
  // OnConfigMapUpdate is called whenever the watched configmap changes
  OnConfigMapUpdated(ctx, v1.ConfigMap)
}

If a resolver implements this interface the framework would start monitoring the resolver's ConfigMap using knative's ConfigMap helpers. For inspiration on exact implementation see Tekton Pipeline's Config store codebase.

Use case

• Allow a resolver developer to support a configmap for storing its admin-level settings.
• Allow an admin deploying a resolver to configure its settings.

Deliverables

Alongside support in code it would also be useful to add:

• instructions to the developer howto
• an update to the copy/pasteable resolver template
• A ConfigMap with timeout setting to the git resolver and bundles resolver