remove S3 SDK deps

[jcstorms1]

Issue #, if available:
#1226
Description of changes:

• Updates S3Event.cs to handle all properties correctly
• Removes SDK.S3 dependency

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[normj]

We don't want to take a direct dependency on Newtonsoft.Json because a lot of users prefer System.Text.Json because it gives a greater cold start benefit. You can avoid referencing Newtonsoft by using DataAnnotation attributes instead of Newtonsoft's JsonProperty attribute.

Here is an example for customizing the names without directly referencing Newtonsoft and supporting System.Text.Json. https://github.com/aws/aws-lambda-dotnet/blob/master/Libraries/src/Amazon.Lambda.LexV2Events/LexV2Message.cs#L45

To avoid complications with System.Text.Json you can drop netstandard2.0 from the Amazon.Lambda.S3.csproj file and have it only target netcoreapp3.1

[jcstorms1]

OK. Thanks for the feedback!

[jcstorms1]

Friendly bump @normj or @ashovlin if you don't mind taking a look at the changes 🙇
cc @ashishdhingra

[normj]

I have pulled down the branch locally and confirm the existing S3 serialization test work. This will be a major version bump but user's migrating should have to do very little to switch. Most likely change a using statement.

[normj]

The PR has been released in version 3.0.0 of the Amazon.Lambda.S3 package. Thank you for your work on this PR!