Add force flag to delete backend

[colerogers]

Add ?force=true to delete a backend with child resources

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 54.18%. Comparing base (eefa8e0) to head (9f1485b).
⚠️ Report is 1574 commits behind head on master.

Files with missing lines	Patch %	Lines
src/gcp/apphosting.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6635   +/-   ##
=======================================
  Coverage   54.18%   54.18%           
=======================================
  Files         347      347           
  Lines       24132    24132           
  Branches     4978     4978           
=======================================
  Hits        13075    13075           
  Misses       9856     9856           
  Partials     1201     1201           

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