Make jwt access token init public

[bbirman]

Ran into this when building one of the templates with dev

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.93%. Comparing base (4ee1195) to head (f7316ea).

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #3787      +/-   ##
==========================================
- Coverage   62.93%   62.93%   -0.01%     
==========================================
  Files         223      223              
  Lines       21295    21295              
==========================================
- Hits        13402    13401       -1     
- Misses       7893     7894       +1     

Components	Coverage Δ
Analytics	84.09% <ø> (ø)
Common	68.96% <ø> (ø)
Core	51.82% <100.00%> (-0.01%)	⬇️
SmartStore	73.70% <ø> (ø)
MobileSync	87.58% <ø> (ø)

Files with missing lines	Coverage Δ
...lesforceSDKCore/Classes/OAuth/JwtAccessToken.swift	83.33% <100.00%> (ø)

... and 2 files with indirect coverage changes