[GO][Client] Multipart/form-data Request Support More Than One File

Created by: icubbon

The Go client will iterate over the files for a multipart/form-data request and add each file to a slice of formFiles. Added a private struct formFile that contains the already defined values of a formFile. A Request now creates an empty slice of formFiles in which files are appended into. The prepareRequest function will now iterate over the slice of formFiles to add each one to the request.

Addresses https://github.com/OpenAPITools/openapi-generator/issues/8322

An example snippet of a schema that has the problem being addressed, note the second file in the multipart/form-data request body:

  "/pet/{petId}/uploadImage":
    post:
      tags:
        - pet
      summary: uploads an image
      description: ""
      operationId: uploadFile
      parameters:
        - name: petId
          in: path
          description: ID of pet to update
          required: true
          schema:
            type: integer
            format: int64
      responses:
        "200":
          description: successful operation
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/ApiResponse"
      security:
        - petstore_auth:
            - "write:pets"
            - "read:pets"
      requestBody:
        content:
          multipart/form-data:
            schema:
              type: object
              properties:
                additionalMetadata:
                  description: Additional data to pass to server
                  type: string
                file:
                  description: file to upload
                  type: string
                  format: binary
                secondFile:
                  description: another file to upload
                  type: string
                  format: binary

@antihax @grokify @kemokemo @jirikuncar @ph4r5h4d

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