Updates StatementExecutionApiClient Execute, Get & GetResultChunk to use source generation

[mellis-ao]

The aim of this is to resolve #290 but also to start the ball rolling on adding RequiresUnreferencedCode attributes & improve awareness of other contributors whether the code is trim safe.

• .Net 6 was removed as it is EOL: https://learn.microsoft.com/en-us/lifecycle/products/microsoft-net-and-net-core
• • Source generation compatibility was not where it needs to be to keep .Net 6 as a target without using pre-processor directives to produce conditional code paths.
• Adds Directory.Build.props to centralise framework targets
• Adds <EnableTrimAnalyzer>true</EnableTrimAnalyzer> to trigger warnings in the code to prompt use of RequiresUnreferencedCode or to use source generated alternatives.
• Places some [RequiresUnreferencedCode] usage on ApiClient and other areas related to the SQL Api.
• Fixes Execute, Get & GetResultChunk methods to use source generation instead of reflection.
• • Also converted to use JsonContent instead of StringContent to avoid the overhead of UTF-16 on strings.

Testing:

Unit tests pass:

Also published a copy of this package to our private Azure Artifacts feed with the fixes and tested in our scenarios. We were able to add <JsonSerializerIsReflectionEnabledByDefault>false</JsonSerializerIsReflectionEnabledByDefault> to our project and not hit any errors (as well was getting results as expected).