Add artifactapi_local_generic resource

Adds a local generic (raw-file) repository resource so arbitrary binaries -- the
bootapi node rootfs tarballs -- can be hosted on artifactapi and managed in
Terraform. The backend already serves generic locals; only the provider surface
was missing.

- resource_local_generic.go: local repo with package_type=generic (mirrors
  local_rpm), registered in the provider.
- Tests + example.

Claude-Session: https://claude.ai/code/session_015ur3i7D2azsMAWTSVABApv

internal/provider/provider.go

@@ -72,6 +72,7 @@ func (p *ArtifactAPIProvider) Resources(_ context.Context) []func() resource.Res
 		NewLocalPyPIResource,
 		NewLocalRPMResource,
 		NewLocalDockerResource,
+		NewLocalGenericResource,
 	}
 }