cloudapi: add support for mirrorlist and metalink repos

Previously, baseurl was required in openapi.yaml. In order to add support
for metalink and mirrorlist repos as well, make all optional, since openapi
does not support mutually exclusive parameters. Instead, enforce this logic
in server.go, and if no repo has been specified, return a 400 bad request error.

internal/cloudapi/openapi.yml

@@ -135,15 +135,22 @@ components:
     Repository:
       type: object
       required:
-        - baseurl
         - rhsm
       properties:
+        rhsm:
+          type: boolean
         baseurl:
           type: string
           format: url
           example: 'https://cdn.redhat.com/content/dist/rhel8/8/x86_64/baseos/os/'
-        rhsm:
-          type: boolean
+        mirrorlist:
+          type: string
+          format: url
+          example: 'https://mirrors.fedoraproject.org/mirrorlist?repo=fedora-33&arch=x86_64'
+        metalink:
+          type: string
+          format: url
+          example: 'https://mirrors.fedoraproject.org/metalink?repo=fedora-32&arch=x86_64'
     UploadRequest:
       type: object
       required: