Support VPC Lattice as an event source

[mbfreder]

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Issue #, if available: #639

Description of changes:

• Added support for VPC Lattice as an event source
• Include a sample app with a cloudformation template to test using SAM CLI

By submitting this pull request

• [ ] I confirm that my contribution is made under the terms of the Apache 2.0 license.
• [ ] I confirm that I've made a best effort attempt to update all relevant documentation.

[deki]

@dmahapatro you may want to take a look as well