Capture common HTTP attributes from API Gateway proxy events in Lambda instrumentor

[finlaysawyer]

Description

This PR addresses part of #1087 to automatically capture common HTTP attributes from API Gateway events in the Lambda instrumentor.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Unit tests below added with events generated via sam local generate-event.
• Deployment of a REST API & Lambda Function with the Lambda Instrumentor enabled. All attributes validated via the console span exporter.
• Deployment of a HTTP API & Lambda Function with the Lambda Instrumentor enabled. All attributes validated via the console span exporter.

Does This PR Require a Core Repo Change?

• No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: finlaysawyer / name: Finlay (576b969)