[DRAFT] enable union backend

[parkan]

this leverages built-in rclone features to allow backends that have one logical root (e.g. internetarchive) to seamlessly aggregate multiple paths into a single preparation

example usage:

RCLONE_CONFIG_IA_TYPE=internetarchive  singularity storage create union --upstreams 'ia:DuckandC1951 ia:0872_Angels_Flight_05_32_34_00' --path ''

this creates an implicit, ephemeral remote of the internetarchive type called ia and aggregates paths specified by --upstreams

[parkan]

why this is a DRAFT: the functionality works as designed but is somewhat specific to the internetarchive remote type as it relies on the implicit creation of a "root" remote (RCLONE_CONFIG_IA_TYPE=internetarchive) on which the upstreams build

for other backend types this becomes potentially non-trivial as the entire remote configuration (auth etc) needs to be specified in env vars (RCLONE_CONFIG_*) and furthermore unions across heterogeneous remote types or other services are untested

all that being said we would really like to see this merged if possible as it is required for SPs to consume preparations created with union sources, to the best of my understanding

the main impediment in my mind is the docstring, which is currently automagically populated from the rclone library; amending it with clarifying comments regarding the above limitations may be sufficient to greenlight this, depending on the maintainer's views, but I'm not entirely sure how to do that

[ianconsolata]

I believe the commits needed to fix the tests can all be pulled from my fork: https://github.com/ianconsolata/singularity/commits/main/. Unfortunately, without maintainer rights I can't update this PR myself.