feat(instance): add support for unified in the backup command

[remyleone]

Community Note

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Relates OR Closes #0000

Release note for CHANGELOG:

[codecov-commenter]

Codecov Report

Merging #2349 (5b82458) into master (664e01f) will decrease coverage by 0.01%.
The diff coverage is 70.27%.

@@            Coverage Diff             @@
##           master    #2349      +/-   ##
==========================================
- Coverage   76.28%   76.27%   -0.02%     
==========================================
  Files         125      125              
  Lines       21053    21088      +35     
==========================================
+ Hits        16060    16084      +24     
- Misses       4454     4461       +7     
- Partials      539      543       +4     

Impacted Files	Coverage Δ
internal/namespaces/instance/v1/custom_server.go	79.80% <66.66%> (-0.40%)	⬇️
internal/namespaces/instance/v1/custom_snapshot.go	70.11% <76.92%> (+1.19%)	⬆️

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[clement-gilbert]

Please add the unified parameter for the "scw instance snapshot create" command

[clement-gilbert]

If the tests cover it, and all tests are ok it LGTM
If also need tests from internal users