Forwarder 3.0.0

[tianchu]

What does this PR do?

Easy install and upgrade:

• Provide CloudFormation quick launch URL for "one-click" install (try me out)
• Host versioned CloudFormation templates in S3 for upgrade (rollback) to a specific version
• Add required IAM permissions to the execution role automatically
• Keep forwarder existing settings, such as environment variables through upgrade
• Group parameters into multiple groups for an organized user interface

Support infrastructure as code:

• Expose all the forwarder settings, such as memory size, timeout and DD_ environment variables as CloudFormation parameters
• Export the forwarder lambda ARN as a CloudFormation output

General improvements:

• Upgrade the Lambda runtime from Python 2.7 to Python 3.7
• Store Datadog API Key in AWS Secrets Manager by default
• Reduce forwarder's own log retention default from forever to 90 days
• Add a release script for release automation
• Use github releases to notify users of new versions
• Attach function.zip to github releases for manual install/upgrade if needed
• Add additional debug logs

Related PRs from the community:
#157
#160
#162
#163
#164
#166
#176
#183