bugfix: Checks for security group association defined independently from vnet

[harkirat22]

This is a proposed fix for #391

• Added new line which will make the rule to only check for scenarios where the vnet has an associated subnet block.
• Added another rule to throw violation for the scenarios where subnet and security group are associated using
  azurerm_subnet_network_security_group_association

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
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[codecov]

Codecov Report

Merging #526 (fcec126) into master (fd73ed6) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #526   +/-   ##
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  Coverage   75.57%   75.57%           
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  Files          94       94           
  Lines        2227     2227           
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  Hits         1683     1683           
  Misses        401      401           
  Partials      143      143