Add artifactapi_local_alpine resource

Adds a local Alpine/apk repository resource mirroring the existing
artifactapi_local_deb and artifactapi_local_rpm locals, so Alpine
package registries can be managed directly via Terraform.

- add localAlpineResource (PackageType alpine, RepoType local) with
  CRUD + import passthrough and model<->API mappers
- register NewLocalAlpineResource in provider Resources()
- bump provider_test resource count to 21 and expected type list
- add local unit tests and README/example entries

examples/resources/artifactapi_local_alpine/main.tf

@@ -0,0 +1,4 @@
+resource "artifactapi_local_alpine" "internal" {
+  name        = "alpine-internal"
+  description = "Internal Alpine package repository"
+}