Add HTTP attributes support for AWS lambda function wrapper

[mateuszrzeszutek]

Related to #1773

This PR implements HTTP server spec for lambdas that use APIGatewayProxyRequestEvent as input.

[anuraaga]

Do you think it makes sense to have a AwsLambdaGatewayProxyTracer extends HttpServerTracer?

[mateuszrzeszutek]

I'd considered choosing different tracers based on the input class, but AwsLambdaTracer has some AWS specific logic (context propagation, faas.* attributes) that I'd need to duplicate in a new tracer (which would extend HttpServerTracer); I'd have to put the AWS logic in a separate helper class if I were to do that.
So out of the two choices: AWS logic in tracer vs HTTP logic in tracer I'd decided to put the HTTP things in the second place; it seemed to me that it'd be easier to add support for other event types this way (share the same tracer class with all the AWS things, separate strategy for additional attributes)

[iNikem]

@anuraaga nobody review this but you :) Or should I just merge it?

[anuraaga]

Oops sorry for missing - I'm a bit worried if the maintenance or http conventions is harder with this approach, e.g., when adding new http sem conventions. But I think it's fine for now anyways