Add AAD auth using azidentity

[shueybubbles]

Fixes #446. Supplants #547 and uses azidentity to acquire tokens.
The basic shape of the change is the same as 547 - it creates a new azuresql driver which wraps the existing driver. The only changes required in core were to make the fedauth connector artifacts public.
I am able to test it manually and with an ADO pipeline run in my organization but I don't know how to automate the E2E using the appveyor setup which is controlled by the project maintainers.
@kardianos please let me know if this satisfies your wishes for protecting the core.
@wrosenuance I think this keeps the spirit of your original change while updating the Azure components. I consulted with other SQL client driver owners to verify that all these authentication methods need to use the "ADAL" workflow.

The readme includes updated usage documentation.

[codecov]

Codecov Report

Merging #698 (fe11108) into master (191d776) will decrease coverage by 0.03%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master     #698      +/-   ##
==========================================
- Coverage   71.20%   71.16%   -0.04%     
==========================================
  Files          24       24              
  Lines        5199     5199              
==========================================
- Hits         3702     3700       -2     
- Misses       1274     1275       +1     
- Partials      223      224       +1     

Impacted Files	Coverage Δ
tds.go	66.33% <85.71%> (+0.36%)	⬆️
accesstokenconnector.go	78.57% <100.00%> (ø)
fedauth.go	71.42% <100.00%> (ø)
token.go	61.22% <0.00%> (-0.79%)	⬇️

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[kardianos]

First glance, this looks good. Pardon the long delay. I'll try to review this today.