Tidy up performance tests

[paulmedynski]

• Tidied up the existing Performance benchmarks.
• Added environment variables to specify config files.
• Removed Newtonsoft.Json as a dependency.
• Simplified csproj a bit.

[Copilot]

Pull Request Overview

This PR tidies up the performance tests by refactoring configuration loading, removing the Newtonsoft.Json dependency, and simplifying project files.

• Introduces a new JSON configuration loader using System.Text.Json.
• Updates performance test configuration for benchmarks and data types across several files.
• Revises the BUILDGUIDE to include instructions for setting up the runner configuration and an empty database.

Reviewed Changes

Copilot reviewed 6 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/Microsoft.Data.SqlClient/tests/PerformanceTests/Config/Loader.cs	Adds a utility to load JSON configs with environment variable overrides.
BUILDGUIDE.md	Updates usage instructions for benchmarks and configuration management.
src/Microsoft.Data.SqlClient/tests/PerformanceTests/Config/DataTypes.cs	Introduces a Load() method to load data types configuration.
src/Microsoft.Data.SqlClient/tests/PerformanceTests/Config/Config.cs	Adds a Load() method for benchmark configuration.
src/Microsoft.Data.SqlClient/tests/PerformanceTests/Program.cs	Refactors config loading and benchmark execution from a static context to an instance context.
src/Microsoft.Data.SqlClient/tests/PerformanceTests/BenchmarkRunners/BaseRunner.cs	Replaces Newtonsoft.Json deserialization with the new Loader API.

Files not reviewed (2)

• src/Microsoft.Data.SqlClient/tests/PerformanceTests/Microsoft.Data.SqlClient.PerformanceTests.csproj: Language not supported
• tools/props/Versions.props: Language not supported

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.92%. Comparing base (f3ae72a) to head (55938e5).
Report is 6 commits behind head on main.

Additional details and impacted files

@@ Coverage Diff @@## main #3230 +/- ##
==========================================
+ Coverage 72.73% 72.92% +0.18% 
==========================================
Files 288 303 +15 Lines 59527 59527 ==========================================
+ Hits 43298 43409 +111 + Misses 16229 16118 -111 

Flag	Coverage Δ
addons	92.58% <ø> (ø)
netcore	75.33% <ø> (+0.14%)	⬆️
netfx	71.51% <ø> (+0.13%)	⬆️

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