Add submission attachment API

[spwoodcock]

This PR

• Anticipates merge of Update deprecated pydantic model Config class to model_config attribute #103, as it uses Pydantic v2 model_config attribute.
• Adds a SubmissionAttachmentService with endpoints for list, get, upload. Does not implement delete, as this is omitted in many services in the lib.
• Added 4 tests for the functionality.

What has been done to verify that this works as intended?

• Tests
  
• Verify with real data / ODK Central (not done yet!)

Why is this the best possible solution? Were any other approaches considered?

• I tried to keep as consistent as possible with existing code.
• There is a FormDraftAttachmentService with a single get and the SubmissionService itself to work from.
• Hopefully it fits in with requirements 🤞- update as needed!

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

• No changes to existing services.
• Additive changes only.

Does this change require updates to documentation? If so, please file an issue here and include the link below.

• No docs updated yet! But they can be once this PR is finalised 👍

Before submitting this PR, please make sure you have:

• included test cases for core behavior and edge cases in tests
• run python -m unittest and verified all tests pass
• run ruff format pyodk tests and ruff check pyodk tests to lint code
• verified that any code or assets from external sources are properly credited in comments