[META 753] Support Partial Transaction for AWS Lambda

[elastic-apm-tech]

See meta issue for the description and details:

• Meta issue: Support Partial Transaction for AWS Lambda apm#753

[trentm]

See this coming lambda extension change: elastic/apm-aws-lambda#384
We should work with and without an extension supporting that.

[trentm]

FWIW, here is the Python PR adding this support: elastic/apm-agent-python#1734

[trentm]

Here is an example of the coming PR for this showing how a timed-out lambda invocation now gets a reported (failed) transaction.