v1.x: Add support for Firebase application default credentials

[kmjennison]

Cherry-pick of #348 onto v1.x.

[vercel]

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[codecov]

Codecov Report

Merging #359 (380e990) into v1.x (9b369ac) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             v1.x     #359   +/-   ##
=======================================
  Coverage   99.59%   99.59%           
=======================================
  Files          25       25           
  Lines         495      497    +2     
  Branches      176      178    +2     
=======================================
+ Hits          493      495    +2     
  Misses          2        2           

Impacted Files	Coverage Δ
src/initFirebaseAdminSDK.js	100.00% <100.00%> (ø)

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