Allow make target customization

[blampe]

Currently, we either ship a fully generated Makefile to providers (bridged) or we leave it entirely up to the provider to implement their own Makefile (native).

There are a number of problems with this:

• Manually maintained Makefiles can break or become stale as the underlying workflows change or evolve. For example as we look to roll out more generalized test sharding (Invert build tags to improve DX #676) we would need to update all native Makefiles to take advantage.
• Our generated Makefiles don't expose any entry points for customization. As a result, we've baked a number options into ci-mgmt.yml which in many cases only apply to one or two providers. This isn't scalable -- it's difficult to maintain and it creates a tight coupling between providers and ci-mgmt.

Instead, ci-mgmt should make some default make targets available to each provider while also giving providers the ability to override those targets should they need to. For example, pulumi-eks should be able to use standard build_go, build_python, build_dotnet targets but also be able to define its own build_nodejs since it's a component provider with unique node requirements.

I've prototyped one approach which seems viable:

• ci-mgmt manages a .mk directory for the provider. This directory includes defaults.mk for default targets, and vars.mk for default variable like PACK=eks.
• Default targets take the form of something like default.build_nodejs:. The defaults.mk file includes a catch-all rule like so:

  include .mk/vars.mk
  
  default.build_nodejs:
     ...
  
  # Delegate any undefined target "foo" to its respective "default.foo" target,
  # if it exists.
  %:
      @$(MAKE) -f defaults.mk "default.$@"

• A provider's Makefile can then contain only the following to automatically pick up any default variables and targets. This should be completely sufficient for the boilerplate to participate in CI workflows:

  include .mk/defaults.mk

• A provider that wished to define custom behavior for itself can then do so as needed:

  include .mk/defaults.mk
  
  lint: # Override default lint target.
      ...