OpenAPI: require all bodies in cross_signing

Signed-off-by: Lukas Lihotzki <lukas@lihotzki.de>

changelogs/client_server/newsfragments/3332.clarification

@@ -0,0 +1 @@
+Clarify that all request bodies are required.

data/api/client-server/cross_signing.yaml

@@ -42,6 +42,7 @@ paths:
           name: keys
           description: |-
             The keys to be published.
+          required: true
           schema:
             type: object
             properties:
@@ -143,6 +144,7 @@ paths:
           name: signatures
           description: |-
             The signatures to be published.
+          required: true
           schema:
             type: object
             title: Signatures