Refactor shim interfaces to not be implementable externally

[VenelinMartinov]

This PR is a pure refactor. The shim interfaces are not intended to be implemented externally. They change often and due to the fact they are public we have kept accruing new variants to keep the public interface backwards compatible which add unnecessary complexity.

This PR adds InternalInterface interface with a private method which is only implemented by internalinter.Internal. This can be embedded in interfaces which are not intended to be implemented outside of the bridge. Implementers in the bridge can embed the implementation of InternalInterface but external users can not.

This means that we are free to add methods to interfaces which are not intended to be implemented externally. This will in turn allow us to delete some of the Schema and Resource variants under shim.

[VenelinMartinov]

This change is part of the following stack:

• Migrate golangci lint #3097
  • Fix PF dynamic type diff error #3098
    • Refactor shim interfaces to not be implementable externally #3100 ◀
      • Required properties with default will now project as optional for SDKv1 providers #3101
        • Implement Dynamic Type serialization #3099

_{Change managed by git-spice.}

[codecov]

Codecov Report

Attention: Patch coverage is 84.21053% with 12 lines in your changes missing coverage. Please review.

Project coverage is 68.79%. Comparing base (2093fe3) to head (5fa7b74).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/pf/internal/schemashim/resource_map.go	72.72%	3 Missing ⚠️
pkg/pf/proto/object.go	0.00%	2 Missing ⚠️
pkg/internal/internalinter/internalinter.go	0.00%	1 Missing ⚠️
pkg/pf/proto/block.go	85.71%	1 Missing ⚠️
pkg/pf/proto/element.go	75.00%	0 Missing and 1 partial ⚠️
pkg/tfshim/sdk-v1/instance_state.go	0.00%	1 Missing ⚠️
pkg/tfshim/sdk-v1/provider.go	75.00%	1 Missing ⚠️
pkg/tfshim/sdk-v1/resource.go	83.33%	1 Missing ⚠️
pkg/tfshim/sdk-v2/resource.go	83.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3100   +/-   ##
=======================================
  Coverage   68.78%   68.79%           
=======================================
  Files         335      336    +1     
  Lines       43560    43575   +15     
=======================================
+ Hits        29962    29976   +14     
- Misses      11887    11888    +1     
  Partials     1711     1711           

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[t0yv0]

Why not just use comments to the purpose? If something out there manages to use these with success, although they are unstable, why be extra strict here and promote additional breakage? That said we are using downcasts aggressively for example on values of type shim.InstanceState so I doubt anything out there is actually going to be broken.

[t0yv0]

And this is public? :) So a determined user could still implement it?

[VenelinMartinov]

No - this is in the internal directory, so no user outside of the bridge can use it.

[t0yv0]

Ah okay !

[VenelinMartinov]

I'd rather we get ahead of it and just prevent any external implementations. We are then completely free to add methods to these interfaces.

I do agree there are unlikely to be any.

[t0yv0]

Since there's likely zero impact I'll approve. If we hear from users we can reconsider and back out.