Implement dependency-less `to_pdb` and `to_cif` functions

[jakublala]

We should provide the user an option to convert to pdb / cif on their local machine, instead of only on Modal.
This will also help reduce the bandwidth necessary for sending things back and forth to/from Modal.
This should not be a major problem, but one never knows what are the demands of users, when for instance sending in jobs on mobile data (with limited network speeds or with a data limit).

This means effectively re-implementing OpenFold's implementation

[jakublala]

This has been partly resolved, as we no longer rely on OpenFold. We could still implement the conversion somehow as a helper function if users truly need to use Modal, and sending in PDBs/CIFs is annoying through the network. Ideally, we should benchmark this. Labeling as low-priority.