Add snapshot method to SiaFile

Christopher Schinnerlrequested to merge
snapshot-siafile into unstable

This MR adds a snapshot method to the SiaFile as described in #3311 (closed).

Depends on !3319 (merged) for the OnComplete method to update the AccessTime of the SiaFile. That allows us to avoid passing a whole SiaFile into the download.

Closes #3311 (closed)

Edited by Christopher Schinnerl

Merge request reports