refactor(storage): adjust storage structure to add backend api

[artek-koltun]

Sort out storage files to corresponding dirs since the number of file is growing, and it is getting harder to comprehend their structure. It was decided to stick to existing storage API separation for frontend/middlend/backend.
The directory structure in cmd mirrors the structure in the root storage libaray package to make it easier to match the related code.
Added frontend cmdline option in a separate commit to prepare to adding backend nvme part.

The inventory, ipsec, evpn code from cmd will be moved to corresponding dirs in the follow-up PRs.

Exemplary cmd set

ss0=$(/dpu storage create frontend nvme subsystem --nqn nqn.2019-06.io.spdk:opicli0 --hostnqn nqn.2019-06.io.spdk:opi-host0 --id subsys0)
ss1=$(/dpu storage create frontend nvme subsystem --nqn nqn.2019-06.io.spdk:opicli1 --id subsys1)
ns0=$(/dpu storage create frontend nvme namespace --subsystem "$ss0" --volume "$vol0" --id namespace0)
tcp0=$(/dpu storage create frontend nvme controller tcp --subsystem "$ss0" --ip "127.0.0.1" --port 4420 --id nvmetcp0)
pcie0=$(/dpu storage create frontend  nvme controller pcie --subsystem "$ss1" --port 0 --pf 0 --vf 0 --id pcie0)
blk0=$(/dpu storage create frontend virtio blk --volume Malloc1 --pf 0 --vf 0  --id blk0)

sleep 10

/dpu storage delete frontend virtio blk --name "$blk0"
/dpu storage delete frontend nvme controller --name "$pcie0"
/dpu storage delete frontend nvme controller --name "$tcp0"
/dpu storage delete frontend nvme namespace --name "$ns0"
/dpu storage delete frontend nvme subsystem --name "$ss1"
/dpu storage delete frontend nvme subsystem --name "$ss0"

Waiting for opiproject/opi-spdk-bridge#837 to pass CI

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6bfed82) 36.84% compared to head (ba85295) 66.43%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #430       +/-   ##
===========================================
+ Coverage   36.84%   66.43%   +29.58%     
===========================================
  Files          19       15        -4     
  Lines        2299     1275     -1024     
===========================================
  Hits          847      847               
+ Misses       1419      395     -1024     
  Partials       33       33               

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[intelfisz]

to create

[artek-koltun]

good catch. Fixed!

[intelfisz]

lgtm, found just one typo

[sandersms]

LGTM - one failing check to resolve