feat: allow configuring openapi server url
Adds a new configuration key in all-in-one.conf named api-server-name. This key allows users to specify the server name openapi should use for validation. Useful in case the CubicWeb instance runs in a sub-path and not the host root.
This key is optional and the default instance base url (using hostname+port) is used if the config is not specified.
Closes #19 (closed)