MIME multipart encapsulation for sending attachments

[kfreezen]

Added a multipart encapsulation for sending attachments to a SOAP server.

Some examples of the current public facing API for attachments:

client = zeep.Client("wsdl-string")
client.service.wsdlService(param0, param1, attachments=[open("attachment-file", "rb")]))

The attachments kwarg attempts to make it as easy as possible for users to attach files to the request. You can pass an attachment any of the following ways

attachments = [open("attachment-file", "rb")]

attachments = [("attachment-content-id.txt", b"Binary encoding. Assumed utf-8")]

attachments = [("attachment-content-id.txt", b"Binary encoding...", "utf-8")]

attachments = [
    AttachmentEncodable(
        name="attachment-encoded.txt",
        data=b"Binary encoding...",
        content_type="utf-8",
        transfer_encoding="8bit",
    )
]