Support OpenAPI specs with .yml file ending (#30)

Currently, only OpenAPI specifications with `.yaml` or `.json` are supported.
YAML files with `.yml` files are equally as idiomatic as `.yaml` files, and should be supported too.

Author: oncilla

internal/openapi_generator.bzl

@@ -150,6 +150,7 @@ _openapi_generator = rule(
             allow_single_file = [
                 ".json",
                 ".yaml",
+                ".yml",
             ],
         ),
         "generator": attr.string(mandatory = True),