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

examples/resources/artifactapi_local_generic/main.tf

@@ -0,0 +1,4 @@
+resource "artifactapi_local_generic" "rootfs_images" {
+  name        = "rootfs-images"
+  description = "Prebuilt node rootfs tarballs (bootapi image provisioning)"
+}