Extend submission.create to support attachments

[yanokwa]

I sometimes have to pull data from a device and send it to a server. I'd like to script that with pyODK but there's no easy way to create a submission with attachments. It'd be nice to have that.

Here's what I'm doing...

#!/usr/bin/env python3

from pathlib import Path
import requests

SUBMISSION_URL = "https://foo.getodk.cloud/v1/key/1234567890/projects/1/submission"
INSTANCES_PATH = Path("/Users/me/Desktop/instances")

def submit_instances(folder_path):
    files = {}

    for file in folder_path.iterdir():
        if file.is_file():
            file_content = file.read_bytes()
            
            if file.suffix == ".xml":
                print(file.name)
                files["xml_submission_file"] = ("instance.xml", file_content)
            else:
                files[file.name] = (file.name, file_content)

    response = requests.post(
        SUBMISSION_URL,
        files=files,
        headers={"X-OpenRosa-Version": "1.0"}
    )

    if response.status_code != 201:
        print(response.text)

for folder in INSTANCES_PATH.iterdir():
    if folder.is_dir():
        submit_instances(folder)

[spwoodcock]

#104 makes a start towards this.

Hopefully it should be a simple update to the SubmissionService .post to include attachment data.
Then we could call SubmissionAttachmentService .upload for each attachment after the main submission POST.