Start teaching the app loader about envvar numbers

[chadwhitacre]

Part of #482, before #524.

We need two passes to parsing, one where we collect configs based on the number in envvars of the form GH_APP_1_FOO, and then another where we organize apps by org slug. This PR stubs out this two-pass structure without yet switching to the new envvars.

[codecov]

Codecov Report

Patch coverage: 95.50% and project coverage change: +0.12 🎉

Comparison is base (3236a12) 84.53% compared to head (ac64786) 84.65%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #523      +/-   ##
==========================================
+ Coverage   84.53%   84.65%   +0.12%     
==========================================
  Files          97       97              
  Lines        2476     2509      +33     
  Branches      474      476       +2     
==========================================
+ Hits         2093     2124      +31     
- Misses        376      378       +2     
  Partials        7        7              

Impacted Files	Coverage Δ
src/webhooks/pubsub/slackNotifications.ts	71.61% <ø> (ø)
src/webhooks/pubsub/index.ts	37.50% <33.33%> (-1.64%)	⬇️
src/config/loadGitHubAppsFromEnvironment.ts	95.12% <94.87%> (-4.88%)	⬇️
src/api/github/getClient.ts	89.28% <100.00%> (+2.61%)	⬆️
src/brain/issueLabelHandler/followups.ts	100.00% <100.00%> (ø)
src/brain/issueLabelHandler/route.ts	97.36% <100.00%> (+0.07%)	⬆️
src/brain/issueLabelHandler/triage.ts	97.82% <100.00%> (+0.04%)	⬆️
src/brain/projectsHandler/project.ts	97.43% <100.00%> (+0.06%)	⬆️
src/config/index.ts	97.82% <100.00%> (-0.18%)	⬇️
src/utils/githubEventHelpers.ts	89.23% <100.00%> (-0.17%)	⬇️
... and 1 more

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