Find Firebase user(s) by federated user id

[mkilmanas]

This is a port of a method that is available in the official node.js sdk, implemented in the analogous way.

[jeromegamez]

The thousands PR, congrats! 😅🎊

Thank you for the contribution! I will have to find out how I can add this without breaking BC due to the changed interface, but that's a general problem 😕

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 11 lines in your changes missing coverage. Please review.

Project coverage is 50.04%. Comparing base (651e64c) to head (3e5b538).
Report is 4 commits behind head on 7.x.

❗ There is a different number of reports uploaded between BASE (651e64c) and HEAD (3e5b538). Click for more details.

HEAD has 5 uploads less than BASE

Flag	BASE (651e64c)	HEAD (3e5b538)
unit	7	2

Additional details and impacted files

@@              Coverage Diff              @@
##                7.x    #1000       +/-   ##
=============================================
- Coverage     90.00%   50.04%   -39.97%     
- Complexity     1444     1447        +3     
=============================================
  Files           140      140               
  Lines          4184     4202       +18     
=============================================
- Hits           3766     2103     -1663     
- Misses          418     2099     +1681     

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