Add python functions.runtime init value and throw error if unset

[colerogers]

Addresses #6774

[codecov-commenter]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (09c2641) 54.30% compared to head (7122eae) 54.29%. Report is 3 commits behind head on master.

Files	Patch %	Lines
src/deploy/functions/runtimes/python/index.ts	0.00%	5 Missing :warning:
src/init/features/functions/index.ts	33.33%	2 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6796      +/-   ##
==========================================
- Coverage   54.30%   54.29%   -0.01%     
==========================================
  Files         350      350              
  Lines       24301    24304       +3     
  Branches     5029     5029              
==========================================
+ Hits        13196    13197       +1     
- Misses       9899     9901       +2     
  Partials     1206     1206              

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