Add interactive prompt when API key is missing

[ml-evs]

Addresses #83 for @jdbocarsly

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 43.14%. Comparing base (2c8035b) to head (78a3826).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/datalab_api/_base.py	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #85      +/-   ##
==========================================
+ Coverage   42.97%   43.14%   +0.17%     
==========================================
  Files           5        5              
  Lines         491      496       +5     
==========================================
+ Hits          211      214       +3     
- Misses        280      282       +2     

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