the main purpose of this PR is to fix the situation where oneOf gets lost in deeply nested allOf schemas, for example:

{
  "allOf": [
    {
      "allOf": [
        {
          "oneOf": [
            { "properties": { "a_foo_one_of_0": { "type": "string" } } },
            { "properties": { "a_foo_one_of_1": { "type": "string" } } }
          ]
        }
      ]
    },
    { "properties": { "a_foo": { "type": "string" } } }
  ]
}

see added comments in merge_schemas.go for explanation

other fixes/improvements:

• includes anyOf as well as oneOf during schema merge
• improves duplicate typename log message (includes path and detailed diff)
• removes unused parameter in mergeOpenapiSchemas