feat(storage-box): use SSH key struct in create opts

[lukasmetzner]

ExampleStorageBoxClient_Create and ExampleStorageBoxClient_Create are two examples, which can be expanded in the func (*StorageBoxClient) [Create] section of the https://pkg.go.dev/ docs.

[codecov]

Codecov Report

❌ Patch coverage is 72.22222% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.53%. Comparing base (be9038e) to head (fee7acd).
⚠️ Report is 1 commits behind head on storage-boxes.

Files with missing lines	Patch %	Lines
hcloud/storage_box.go	72.72%	2 Missing and 1 partial ⚠️
hcloud/schema_gen.go	71.42%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##           storage-boxes     #754   +/-   ##
==============================================
  Coverage          78.52%   78.53%           
==============================================
  Files                 59       59           
  Lines               5603     5619   +16     
==============================================
+ Hits                4400     4413   +13     
- Misses               911      912    +1     
- Partials             292      294    +2     

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