Add tsa config file to enable validation pipeline (#8666)

Context
MSBuild had been added to the code validation pipeline but was missing the tsa config file. Per the instructions here and after talking to Matt, adding this file

Changes Made
Added a tsa config file by copying from SDK

Testing
Not sure how to test this. I pushed an internal build but the checks didn't run as I'm told they run nightly in a separate pipeline.

Notes
Validation pipeline: https://dev.azure.com/dnceng/internal/_build/results?buildId=2157183&view=results

eng/sdl-tsa-vars.config

@@ -0,0 +1,11 @@
+-SourceToolsList @("policheck","credscan")
+-TsaInstanceURL https://devdiv.visualstudio.com/
+-TsaProjectName DEVDIV
+-TsaNotificationEmail dotnetdevexcli@microsoft.com
+-TsaCodebaseAdmin REDMOND\marcpop
+-TsaBugAreaPath "DevDiv\NET Tools\MSBuild"
+-TsaIterationPath DevDiv
+-TsaRepositoryName DotNet-msbuild-Trusted
+-TsaCodebaseName DotNet-msbuild-Trusted
+-TsaOnboard $True
+-TsaPublish $True