add SubmittableDeal type

[NikolasHaimerl]

We are currently not using a specific type when submitting deals through the deal observer backend.
In two recent discussions (Here and here), the need for a specific type for submittable deals has come up.

This PR suggests a new type for deals that are found to be submittable and used throughout the submitting process of the deal observer.
This solves the issue of the use of the any type when activating type checking and also delivers clarity on the type returned by various functions during the submitting process.

[bajtos]

The proposal in this pull request is consistent with what we are doing elsewhere in this project, I don't see any obvious issues besides one minor comment below.

[pyropy]

I think we can improve type signature to make it simple and more correct.

[pyropy]

Good job! 👏🏻