feat: AUTH_REMOTE_USER_ENV_VAR config key for auth REMOTE_USER type

[dpgaspar]

Description

A simple solution for: #2014

Add a new config key AUTH_REMOTE_USER_ENV_VAR to allow for users to set their own WSGI env var that holds the current username.

ADDITIONAL INFORMATION

• Has associated issue:
• Is CRUD MVC related.
• Is Auth, RBAC security related.
• Changes the security db schema.
• Introduces new feature
• Removes existing feature

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (9123fcb) 79.57% compared to head (23cdc5e) 79.71%.

Files	Patch %	Lines
flask_appbuilder/security/views.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2193      +/-   ##
==========================================
+ Coverage   79.57%   79.71%   +0.13%     
==========================================
  Files          72       72              
  Lines        8985     8990       +5     
==========================================
+ Hits         7150     7166      +16     
+ Misses       1835     1824      -11     

Flag	Coverage Δ
python	79.71% <83.33%> (+0.13%)	⬆️

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