Capabilities Endpoint methods missing "OPTIONS"

[m-mohr]

For each endpoint in GET / the methods can be specified. The enum only lists: "GET" "POST" "PATCH" "PUT" "DELETE".
The description states: "It is OPTIONAL to list OPTIONS as method (see the CORS section)."
That means we need to add "OPTIONS" to the enum, otherwise validation would fail if OPTIONS is included in the methods.