[Wsdl] Handle schema property of type array with oneOf items and other minor updates/fixes

[adessoDpd]

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Description of the PR
This small PR adds handling of specific case: Schema with property of type array with oneOf items. In case of this example following WSDL-construct is created.

components:
   schemas:
      fruit:
         title: fruit
         properties:
            color:
               type: string
            Fruitarray:
              type: array
              items:
                oneOf:
                  - $ref: "#/components/schemas/apple"
                  - $ref: "#/components/schemas/banana"

<xs:complexType name="Fruit">
  <xs:sequence>
    <xs:element minOccurs="0" name="color" type="xs:string" />
    <xs:element minOccurs="0" maxOccurs="unbounded" name="apple" type="schemas:Apple" />
    <xs:element minOccurs="0" maxOccurs="unbounded" name="banana" type="schemas:Banana" />
  </xs:sequence>
</xs:complexType>

Some minor updates/fixes has been added as well in this PR since the handling of the oneOf-case was rather small:

• Uppercase first letter of basetype/datatype of responses (in case of models) to handle the case if schema name was lowercase
  in openapi spec to have a correct reference to the wsdl-complextype:

...
<xs:complexType name="Get_ResponseMessage">
  <xs:sequence>
    <xs:element minOccurs="1" name="Fruit" type="schemas:Fruit">  // instead of  type="schemas:fruit" = schema not found
      ...

• Remove unnecessary vendor-extension which checked if a openapi model was just an enum, since for this case "isEnum" already existed.
• Shortening the code in some places for upper-/lowercasing the first letter using substring
• If response is a file (isFile) type xs:string is now used instead of xs:anyType

I am aware that "oneOf" is still barely handled in this generator like in some other generators, so it is definitely something that needs to be done in the future.

[wing328]

Looks good 👍

I'll update the samples and docs after merging this PR.