Add v0.9.0 Release Notes

.github/releases/v0.9.0.md

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+## Enhancements
+
+- Request serialization / deserialization is now handled by Lambdajection instead of the serializers provided by Amazon's Runtime Support. This implementation is purely System.Text.Json-based.
+- X-Ray Active Tracing can now be easily enabled by adding a PackageReference to the new Lambdajection.Tracing package.
+- Enables .NET 6 Support and drops .NET 5 support.
+- SNS Event Handling is now built in to Lambdajection.  Add the Lambdajection.Sns package to your project and use the SnsEventHandlerAttribute to denote Lambdas that are sourced from SNS events.  The lambda handler will be invoked once for each SNS record received.
+  - A model for CloudFormation Stack Notifications is included in the Lambdajection.Sns package for processing stack notification events.
+- The lambda output for Custom Resources is now the full response that would've been sent to CloudFormation, rather than just the output data.