Cache the whole client

[chadwhitacre]

We've had this comment forever:

Not sure if we can cache the octokit instance - installation tokens expire
after an hour, but octokit client may be able to handle this properly.

I found docs that say the octokit client does handle this properly!

This is preparatory to adding support for configuring more than one client to support multi-org for #482.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.04 ⚠️

Comparison is base (4de769c) 82.74% compared to head (8efab1c) 82.71%.

❗ Current head 8efab1c differs from pull request most recent head 692886b. Consider uploading reports for the commit 692886b to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #489      +/-   ##
==========================================
- Coverage   82.74%   82.71%   -0.04%     
==========================================
  Files          96       96              
  Lines        2486     2487       +1     
  Branches      479      479              
==========================================
  Hits         2057     2057              
- Misses        423      424       +1     
  Partials        6        6              

Impacted Files	Coverage Δ
src/api/github/getClient.ts	0.00% <0.00%> (ø)

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[hubertdeng123]

looks good!🚀