Cache apple public key for the case it fails to fetch again

[davimacedo]

@dplewis what do you think?

[dplewis]

I'll do a PR to support private keys as well.

[codecov]

Codecov Report

Merging #5848 into master will decrease coverage by 0.03%.
The diff coverage is 57.14%.

@@            Coverage Diff             @@
##           master    #5848      +/-   ##
==========================================
- Coverage    93.7%   93.66%   -0.04%     
==========================================
  Files         148      148              
  Lines       10310    10315       +5     
==========================================
+ Hits         9661     9662       +1     
- Misses        649      653       +4

Impacted Files	Coverage Δ
src/Adapters/Auth/apple.js	89.65% <57.14%> (-10.35%)	⬇️
src/Adapters/Auth/httpsRequest.js	95.23% <0%> (-4.77%)	⬇️

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