Reduce number of required properties

[nokome]

The initial versions of YAML schema definitions listed properties which should really be optional, as required. This was partly a mistake and partly blindly following what had already been setup in oxa-types.json.

Before

> node packages/oxa/dist/cli.cjs validate
{"type":"Document","children":[]}

<stdin>: invalid
  /: must have required property 'metadata'
  /: must have required property 'title'

After

> node packages/oxa/dist/cli.cjs validate
{"type":"Document","children":[]}
<stdin>: valid