add external to apply and default requirement type

[mike0sv]

No description provided.

[codecov]

Codecov Report

Merging #196 (0adc3ba) into main (d5525e8) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #196      +/-   ##
==========================================
- Coverage   89.55%   89.51%   -0.04%     
==========================================
  Files          74       74              
  Lines        4911     4912       +1     
==========================================
- Hits         4398     4397       -1     
- Misses        513      515       +2     

Impacted Files	Coverage Δ
mlem/cli/apply.py	95.23% <ø> (ø)
mlem/api/commands.py	88.88% <100.00%> (ø)
mlem/core/requirements.py	95.54% <100.00%> (+0.01%)	⬆️
mlem/contrib/heroku/meta.py	82.85% <0.00%> (-2.86%)	⬇️

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[aguschin]