Support for DiagnosticSource

[bruno-garcia]

As discussed on a call while reviewing #1149:

• Create a new package Sentry.Diagnostics.DiagnosticSource
• New package will:
  • depends on System.Diagnostics.DiagnosticSource
  • target only ns2.0, ns2.1, net461 (newer targets already include S.D.D
• Sentry package will include all ../Sentry.Diagnostics.DiagnosticSource/**/*.cs files when targeting nca3.0 or higher
• Unit tests under S.D.D would also be included in Sentry.Tests
• Packages that will depend on Sentry.Diagnostics.DiagnosticSource:
  • Sentry.AspNet
  • Sentry.Extensions.Logging when targeting ns2.0. #ifdef to enable the listeners on that TFM
• Document the package, particularly:
  • This package is not needed when using Sentry with .NET Core 3 or higher.
  • This package is already brought in (you don't need to add) when using:
    • Sentry.Extensions.Logging
    • Sentry.AspNet
    • Sentry.AspNetCore
  • You can add this package to get:
    • Spans for queries executed with System.Data.SqlClient
    • ... (we can add HTTP requests too without the need of the HttpMessageHandler)
• Helper method to add all listeners to be called by Sentry itself on nca3.0 or higher, or users of S.D.D by hand. Such as: SentrySdk.Init(o => o.AddDiagnosticListeners()

[bruno-garcia]

Done through #1154.
Closing this as done. Some final touches like docs will be tracked through #1172