Move AAD/Entra Authentication into new Azure package - #3680

- Removed addition of PDBs from <AllowedOutputExtensionsInPackageBuildOutputFolder>.
- Removed MDS package ref dependency on Abstractions until pipelines are ready.
- Renamed AbstractionsPackage to Abstractions in targets.
- Updated BUILDGUIDE based on project ref behaviour.
- Added feature branches to CI pipeline triggers.
- Added missing/incomplete paths to the trigger.
- Added dev/* branches to the CI triggers so PRs that target other dev/ branches can run CI.
- Added missing MdsPackageVersion property to signed build pipeline job.
- Commented-out failing NuGet tool installer task.
- Re-ordered Guardian analysis step _before_ build step to avoid clobbering versioned DLLs.
- Added MDS package version to AKV build/package steps.
- Restored AKV nuspec ReferenceType property for AKV Official builds.
- Explicitly building tooling before analysis.
- Fixing validation steps to match XML props files.
- Added PR automation triggers, and documented other pipeline sections.
- Added ReferenceType throughout the MDS/AKV CI build steps.
- Added NuGet.config update to main CI build step for Package reference mode.
- Swapped Abstractions download and NuGet.config update to ensure packages/ exists before attempting to re-configure NuGet.
- Clean target no longer removes packages/
- Uncommented package refs to Abstractions.
- Added separate MDS and AKV project builds to support Package mode.
- Added $ReferenceType$ property to MDS .nuspec like it is for AKV.
- Removed obsolete version variables from merge conflict.
- Adding $ReferenceType$ back to AKV nuspec.
- Moved SqlAuthenticationProvider and friends into Abstractions.
- Moved ActiveDirectoryAuthenticationProvider into a new Azure package.
- Moved a bunch of docs into Abstractions and Azure projects.
- Updated Abstractions classes to be abstract with minimal implementation.
- Added back the implementations to MDS.
- Re-worked some of the docs.
- SqlAuthentication* classes separated out into Base classes in Abstractions.
- Updated docs accordingly.
- MDS compiles.
- Some Azure package code moved, but not compiling.
- Added an exception class for authentication errors.
- Enabled XML doc compilation and validation.
- Updated MDS to handle SqlAuthenticationProviderException instead of MSAL exceptions.
- Removed Azure.Identity from MDS.
- Got existing tests compiling using the new Azure package.
- Added Azure package build targets.
- Updated docs related to ReferenceType
- Got all projects properly adhering to Package reference type.
- Removed TestMicrosoftDataSqlClientVersion in favour of MdsPackageVersion.
- Fixed tools restore from build.proj.
- Fixed missing using for dummy auth provider.
- Fixed obsolete warnings for password auth.
- Moved SqlAuthenticationParameters entirely into Abstractions, avoiding an empty base class.
- Moved SqlAuthenticationToken entirely into Abstractions.
- Moved SqlAuthenticationProvider entirely into Abstractions.
- Removed SqlAuthenticationProviderBase.
- Exposed SqlAuthenticationProviderManager as a public API in MDS.
- Moved SetProvider() and GetProvider() to the Manager where they should have been all along (Breaking Change).
- Added catch-all exception handlers in prep for proper analysis and specific exception handling.
- Some improvements to avoid meaningless nulls.
- Added PR/CI pipeline support for the Azure package.
- Fixed Abstractions download path.
- Updated SqlAuthenticationProviderException to use Method instead of Action.
- Fixed NRE in SqlException.
- Fixed SqlAuthenticationProviderException use in MDS.
- Added chaining Add() to SqlErrorCollection.
- Added a useful message when _fedAuthToken is null after attempting to acquire a token.
- Fixed auth provider error handling to mimic the old code.
- Removing unnecessary database checks that are hanging forever.
- Fixed timestamp truncation from long to uint causing cached tokens to be erroneously expired.

…r to maintain API for now.

- Addressed review comments.

…qlClient-Package runs.