Allow sni version override for testing

[mdaigle]

When we make changes to SNI, it's important to test the changes in combination with MDS to avoid regressions. This PR adds the ability to trigger a pipeline run with an overridden SNI version pulled from the internal validation feed in ADO. This functionality is only available to pipeline runs within the ADO.Net project in ADO. Pipeline runs within the public project do not have access to the internal feed and will fail if an override version is set.

Proof that runs fail in public pipelines when sni override is set:
Project: https://sqlclientdrivers.visualstudio.com/public/_build/results?buildId=109913&view=results
Package: https://sqlclientdrivers.visualstudio.com/public/_build/results?buildId=109914&view=results

[paulmedynski]

Looks good, just a few nitpicks.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.80%. Comparing base (17cb0b0) to head (2702322).
Report is 26 commits behind head on main.

Additional details and impacted files

@@ Coverage Diff @@## main #3184 +/- ##
==========================================
- Coverage 72.96% 72.80% -0.16% 
==========================================
Files 283 285 +2 Lines 58997 59137 +140 ==========================================
+ Hits 43048 43057 +9 - Misses 15949 16080 +131 

Flag	Coverage Δ
addons	92.58% <ø> (ø)
netcore	75.53% <ø> (-0.20%)	⬇️
netfx	71.15% <ø> (-0.21%)	⬇️

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[benrr101]

I think you and I should have a bit of a conversation about when and how the handful of components need to be tested together (MDS, SNI, AKV, MSS, ETC, WTF, BBQ). I want to get a better idea what depends on what and how the pipelines should be constructed to handle those situations. It's nothing that needs to block this specific PR, but since I think we will want to rebuild this pipeline as part of other engineering goals, I think we'll want to discuss it at some point.