fprime-gds: default file downlink directory can overwrite files

[timcanham]

F´ Version	v3.4.3
Affected Component	n/a

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Feature Description

Have GDS created a time-tag directory for file downlinks.

Make the default be logs/<time tag>/fprime-downlink to match telemetry and events.

Also, make a separate argument for uplink and downlink. It makes sense to automatically separate downlinked files, but uplink may wish to be in a common store.

Rationale

The default /tmp/username (or even overridden with --file-storage-directory) directory for file downlinks can cause new instances to overwrite older files, or cause parallel instances to clobber each other.

[Joshua-Anderson]

Also discussed in #2658 (comment)