Improve support for enums

[jloleysens]

Specific to @kbn/config-schema conversion logic in router-to-openapispec package, given a schema like:

  schema.oneOf([
    schema.literal('available'),
    schema.literal('degraded'),
    schema.literal('unavailable'),
    schema.literal('critical'),
  ]);

Give OAS like:

{
  "enum": [
    "available",
    "degraded",
    "critical"
  ],
  "type": "string"
}

Rather than

                   "anyOf": [
                      {
                        "enum": [
                          "available"
                        ],
                        "type": "string"
                      },
                      {
                        "enum": [
                          "degraded"
                        ],
                        "type": "string"
                      },
                      {
                        "enum": [
                          "unavailable"
                        ],
                        "type": "string"
                      },
                      {
                        "enum": [
                          "critical"
                        ],
                        "type": "string"
                      }
                    ]

Notes

• This should still produce anyOf structures where needed, this is more a matter of finding a way to handle actual enums as enums