feat: allow settings config options from environment

[0xd3e]

This allows setting credentials and other options from environment variables like the CLI does.
Right now the environment variables take precedence. If profile is set and the env variable SCW_ACCESS_KEY and SCW_SECRET_KEY, the env variables will be used.

If this should be the other way around, please let me know.
I don't know if the presence of the env variables will affect the modules.

Fixes: #56

[remyleone]

Thanks a lot @0xd3e for you contribution :) We will iterate on it

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 12.50000% with 7 lines in your changes missing coverage. Please review.

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Files with missing lines	Patch %	Lines
plugins/inventory/scaleway.py	12.50%	7 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #57   +/-   ##
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  Coverage        ?   32.45%           
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  Files           ?       64           
  Lines           ?     4126           
  Branches        ?      813           
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  Hits            ?     1339           
  Misses          ?     2787           
  Partials        ?        0           

Flag	Coverage Δ
sanity	32.45% <12.50%> (?)

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