Ability to integrate external data prep tools with singularity

[anjor]

https://filecoinproject.slack.com/archives/C033QS35MFA/p1682671382057909
anjor/go-fil-dataprep#19

I would like to be able to use singularity to make deals with car files prepared by other tools (such as go-fil-dataprep).

My guess is this will just require an api (or some mechanism) on the singularity side to insert the metadata about the prepared files into the singularity database.

[xinaxu]

addressed in 1755cd3