Alter FreeSurfer subjects to include session labels

[mgxd]

A short summary of what you would like to see in NiBabies.

As we are moving to session-based processing, multiple runs of FreeSurfer can clash. We should probably alter subject_id to include the session information, like is mentioned in:
https://surfer.nmr.mgh.harvard.edu/fswiki/BIDS

Do you have any interest in helping implement the feature?

Yes!

Add any additional information or context about the request here.

No response

[oesteban]

@yasseraleman look at this