Refactor to enable multiple backends

[jakublala]

This is going to be a major refactor that will hopefully make everything more scalable down the line.

We will go for an implicit context creation and deletion, similar to what BAGEL already does when using boileroom. Thus a model will be used as:

from boileroom import ESMFold
model = ESMFold(backend="apptainer")
model.fold(sequences)

Where the backend could be any of the apptainer, docker, conda, or modal.

Then there's going to be an ExecutionBackend as a base class, communicating through asyncio -- through BackendResponse and BackendRequest.