Implemented the passport-jwt secretOrKeyProvider

[askides]

Hello, hope you are well :)

In this PR i've implemented the secretOrKeyProvider method of the passport-jwt library.

The problem is, if i have many users, everyone with a jwt credential associated, is impossible to make requests, because jwt policy permit to set only one secret, or only one key file for all the requests, so if i sign every jwt with its secret, the gateway respond unauthorized.

This implementation permit to the jwt policy to check the secret for each user with the JWT credentials set.

So, now if there aren't set the params secretOrPublicKey and the secretOrPublicKeyFile, this check is triggered.

Renny.

[askides]

Updated also test cases for this PR.

[codecov[bot]]

Codecov Report

Merging #986 into master will decrease coverage by 0.09%. The diff coverage is 91.66%.

@@            Coverage Diff            @@
##           master     #986     +/-   ##
=========================================
- Coverage   88.97%   88.87%   -0.1%     
=========================================
  Files         137      137             
  Lines        3755     3767     +12     
=========================================
+ Hits         3341     3348      +7     
- Misses        414      419      +5

Impacted Files	Coverage Δ
lib/policies/jwt/index.js	100% <ø> (ø)	:arrow_up:
lib/policies/jwt/jwt.js	90.47% <91.66%> (+0.47%)	:arrow_up:
lib/config/config.js	89.36% <0%> (-4.26%)	:arrow_down:

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