feat: update to terraform plugin framework

[fabiomatavelli-f3]

Summary

Migrate the provider from the deprecated terraform-plugin-sdk (v1) to the modern terraform-plugin-framework.

Motivation

Dependabot attempted to update github.com/hashicorp/go-getter to v1.7.9 (#68), which transitively pulled in github.com/spf13/afero v1.9.2. The newer afero version added a Chown method to the afero.Fs interface, but the terraform-plugin-sdk v1 internally vendors a snapshotFS type that does not implement this new method — causing a compile-time error:

snapshotFS does not implement afero.Fs (missing method Chown)

Since terraform-plugin-sdk v1 is unmaintained and will never receive a fix, the proper solution is to migrate to the terraform-plugin-framework, which does not depend on afero at all.

Changes

Source files

• main.go — Uses providerserver.Serve() from the framework
• githubfile/provider.go — Rewritten using the provider.Provider interface (Metadata, Schema, Configure, Resources, DataSources)
• githubfile/resource_file.go — Rewritten using the resource.Resource interface (Create, Read, Update, Delete, ImportState) with extracted testable business logic functions
• githubfile/structure_file.go — Simplified to the file struct and parseFileID helper
• githubfile/provider_test.go — Updated to use providerserver.NewProtocol6WithError and tfprotov6
• githubfile/resource_file_test.go — Updated to use ProtoV6ProviderFactories and framework-compatible testing

Dependencies

• Added: terraform-plugin-framework, terraform-plugin-go, terraform-plugin-testing
• Removed: terraform-plugin-sdk v1 (direct), terraform-provider-github/v5 (direct), afero, go-getter, and ~1M lines of vendored code
• Removed: vendor/ directory (no longer needed)

Behavioral notes

• All provider attributes, resource attributes, and their semantics remain unchanged
• Import ID format remains owner/repo:branch:path
• ForceNew behavior preserved via stringplanmodifier.RequiresReplace()
• Archived repository detection on delete is preserved
• GPG signing support is preserved
• Environment variable fallbacks (GITHUB_TOKEN, GITHUB_EMAIL, etc.) are preserved

Supersedes

This PR supersedes #68 (Dependabot go-getter bump) by removing the dependency entirely.