Cleanup | Centralise AppContext switches

[edwardneal]

Description

We try to centralise all access to SqlClient's AppContext switches via the LocalAppContextSwitches class. There are currently a few cases where we don't do this though:

• netcore's globalization invariant mode detection lives in SqlConnection
• Switch.Microsoft.Data.SqlClient.TruncateScaledDecimal is handled by TdsParser on both targets
• Switch.Microsoft.Data.SqlClient.UseManagedNetworkingOnWindows is handled by TdsParserStateObjectFactory

The first port of call is to relocate all of these to the existing LocalAppContextSwitches class. This slightly helps in the merge of SqlConnection.

I've also added an ILLink.Substitutions.xml resource to the output. This is a trimming improvement, and is documented here. It allows referencing applications to specify a RuntimeHostConfigurationOption to set an AppContext switch, which the IL trimmer will reference and remove the entire managed or native SNI. There's a little more work to come on this once trimming is ready.

For comparison purposes, this is modelled in part on CsWinRT:

• ILLink.Substitutions.xml
• Underlying FeatureSwitches type
• Usage of RuntimeHostConfigurationOption

I've used sizoscope to verify that I can use RuntimeHostConfigurationOption, publish an application and see the corresponding SNI's types are removed from the output.

Issues

Partially relates to #1261 and #1942.

Testing

Unit tests pass locally.

[paulmedynski]

/azp run

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