feat(fwprovider): export the provider implementation for embedding - #434

Adds a public fwprovider package that hands out the provider implementation, so Go programs can embed the provider in-process instead of running it as a plugin binary.

Motivation

In #354 I asked for a Crossplane provider for Coder. Crossplane's Upjet generates one directly from a Terraform provider's schema, and it can drive that provider two ways:

• Terraform CLI mode. Upjet bundles the terraform binary and the provider binary into its controller image, then runs terraform against a workspace per managed resource, which in turn launches the provider plugin. It works against any provider and needs no cooperation from upstream.
• In-process mode. Upjet calls the provider's provider.Provider implementation directly through its plugin-framework client. No terraform binary, no provider binary, no subprocess per reconcile.

In-process mode needs an importable provider.Provider. internal/provider is unimportable outside this module, so today the only route is to fork this repo and add the export in the fork. That is exactly what Upbound does for the providers that keep theirs internal — terraform-provider-aws/xpprovider and terraform-provider-azurerm/xpprovider, both wired in through a replace directive. Providers that expose theirs upstream need no fork at all: terraform-provider-google has google/fwprovider, terraform-provider-keycloak has provider.

I would rather not maintain a fork of this repo that has to be re-synced on every release, and I doubt you want one circulating either.

What changed

A single public package delegating to the existing constructor:

func New(version string) func() tfprovider.Provider {
	return provider.New(version)
}

Returning the factory rather than an instance keeps the signature identical to provider.New, so it still drops straight into providerserver.Serve; embedders call it to get an instance.

Kept deliberately small:

• No new dependencies — terraform-plugin-framework is already required.
• No behaviour change and no second code path. It is a one-line delegation, so it cannot drift, and a signature change to provider.New breaks the build here too.
• internal/ stays internal. This exports the provider surface only, not resource implementations, codersdk plumbing, or helpers.
• main.go is untouched.
• A test asserts the factory yields the provider main.go serves, with TypeName, version, resources and data sources all wired.

I named it fwprovider after the terraform-provider-google precedent rather than Upbound's Crossplane-specific xpprovider, on the grounds that this is an export you own rather than one living in a downstream fork. Happy to rename it, relocate it, or narrow the signature.

Context

I have built an Upjet-based Crossplane provider covering all 15 resources, exposing both cluster-scoped and namespaced Crossplane v2 APIs, with a scheduled job that regenerates and opens a PR when a release lands here. It runs in Terraform CLI mode today, because that is the only mode available without forking this repo.

I took the liberty of starting the implementation rather than just filing the request, so there is something concrete to react to. If you would like the Crossplane provider itself to live under the Coder org, I am happy to donate it; equally happy to keep maintaining it out-of-tree if you would rather not adopt another repo. Either way this export stands on its own for anyone embedding the provider.

Refs #354