Allow for local dns resolution with a custom dialer

[stephaniehingtgen]

If the custom dialer updates the transport to proxy the mssql connection, and only the proxy can reach the network that mssql is running in, then any local DNS resolution will fail.

To address this, this PR adds an attempt to dial the connection via the custom dialer, if the DNS resolution fails.

[codecov[bot]]

Codecov Report

Merging #779 (d1ee717) into master (6fb7982) will decrease coverage by 0.14%. The diff coverage is 28.57%.

:exclamation: Current head d1ee717 differs from pull request most recent head e0b26b5. Consider uploading reports for the commit e0b26b5 to get more accurate results

@@            Coverage Diff             @@
##           master     #779      +/-   ##
==========================================
- Coverage   71.24%   71.11%   -0.14%     
==========================================
  Files          24       24              
  Lines        5405     5411       +6     
==========================================
- Hits         3851     3848       -3     
- Misses       1307     1313       +6     
- Partials      247      250       +3     

Impacted Files	Coverage Δ
tds.go	65.14% <28.57%> (+0.01%)	:arrow_up:

... and 2 files with indirect coverage changes

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[denisenkom]

Thank you for this pull request. Can you add test coverage for this change?

[stephaniehingtgen]

Thank you for this pull request. Can you add test coverage for this change?

Definitely! Updated :)