Serverless: Lambda .Net Core 2.1 Support replacing EOL Core 2.0

[RafaelGorski]

This is a Feature Proposal

Description

Adding support to .Net Core 2.1 runtime on template files.

For bug reports:

• What went wrong?
  Microsoft announce the end of life of AWS Core 2.0
  Reference: https://blogs.msdn.microsoft.com/dotnet/2018/06/20/net-core-2-0-will-reach-end-of-life-on-september-1-2018/

• What did you expect should have happened?
• What was the config you used?
• What stacktrace or error message from your provider did you see?

For feature proposals:

• What is the use case that should be solved. The more detail you describe this in the easier it is to understand for us.
• If there is additional config how would it look

Similar or dependent issues:
N/A

Additional Data

• Serverless Framework Version you're using:
• Operating System: Windows
• Stack Trace:
• Provider Error messages:

[jdonohoo]

I started working on this, and then double checked here before submitting a pull request.

I see slang25 has a pull request ready to be reviewed. (I approved and added comments that we might want to consider).