fix(instance): display sbs volumes' size in instance image list details

[Mia-Cross]

The command scw instance image list now fetches volume information from the SBS API to display the correct size of the underlying snapshots.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 71.42857% with 6 lines in your changes missing coverage. Please review.

Project coverage is 54.67%. Comparing base (5bf53e0) to head (888c0c3).
Report is 652 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/namespaces/instance/v1/custom_image.go	71.42%	4 Missing and 2 partials ⚠️

❗ There is a different number of reports uploaded between BASE (5bf53e0) and HEAD (888c0c3). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (5bf53e0)	HEAD (888c0c3)
	2	1

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #4766       +/-   ##
===========================================
- Coverage   75.45%   54.67%   -20.78%     
===========================================
  Files         202      315      +113     
  Lines       44323    80629    +36306     
===========================================
+ Hits        33444    44087    +10643     
- Misses       9653    35135    +25482     
- Partials     1226     1407      +181     

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