Missing header normalization on AWS lambda

[xrmx]

Describe the bug:

It looks we are missing some case normalization in the AWS Lambda instrumentation. Even if headers are documented as being already lowercase in AWS docs, it looks like there are cases where this is not.

Nodejs agent already handles the issue here:
https://github.com/elastic/apm-agent-nodejs/blob/2b69514d0b2c97bb974346cce975cf0cc958d559/lib/lambda.js#L705-L725

Outcome of this is that we are not getting any traceparent or tracestate header and so we are not able to correlate transactions.

To Reproduce

1. run apm agent instrumentation for aws lambda
2. inspect the event and see headers are not normalized
3. see that we are not able to get traceparent or tracestate header

Environment (please complete the following information)

• OS: [e.g. Linux] aws lambda
• Python version: -
• Framework and version [e.g. Django 2.1]: -
• APM Server version: -
• Agent version: latest