AAD Authentication using AccessToken

[paulmey]

Although it leaves room for improvement with respect to convenience from the DSN, this PR implements AAD authentication through access tokens.
To test this implementation, create a SQL Azure database and set yourself as the AAD admin for the database. Then run

az login # if not already logged in
at=$(az account get-access-token --resource https://database.windows.net/ --q accessToken -o tsv)
export SQLSERVER_DSN="sqlserver://srv.database.windows.net?database=db&accesstoken=$at"
go test -v

To use a service principal, grant the service principal access to the database as described in the docs.

At least partially fixes #446

[codecov]

Codecov Report

Merging #546 into master will decrease coverage by 0.19%.
The diff coverage is 61.29%.

@@            Coverage Diff            @@
##           master     #546     +/-   ##
=========================================
- Coverage    68.9%   68.71%   -0.2%     
=========================================
  Files          22       23      +1     
  Lines        5068     5187    +119     
=========================================
+ Hits         3492     3564     +72     
- Misses       1370     1410     +40     
- Partials      206      213      +7

Impacted Files	Coverage Δ
conn_str.go	100% <ø> (ø)	⬆️
token.go	54.54% <0%> (-1.37%)	⬇️
tds.go	61.03% <66.66%> (+0.69%)	⬆️
accesstokenconnector.go	80% <80%> (ø)

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[paulmey]

I'll try to raise code coverage a bit

[kardianos]

This looks really good. I only have nits.

The only real change I really want is to put a go.mod file in the example folder or at a min in the example/azure-managed-identity folder, so the mssql module doesn't depend on adal.

[paulmey]

I added a go.mod in the specific example folder and fixed most of the nits. The go.mod in the example need to be removed to make the example work until this gets merged, but hopefully that's only going to be temporary.