Added integration tests for http adapter

[ferdingler]

Description of changes:
Added integration tests for the HTTP adapter using an HTTP mock server. The mock server simulates the web application and it actually spins up a Hyper server locally so the tests are excercising the majority of the adapter logic, except for the part where we register as a Lambda extension (see note 2 below).

Note
I made a minor change to the AdapterOptions struct to make fields public so that we could initialize the struct from within the tests. Using environment variables for the tests wasn't an option as by default all tests run in parallel and modifying env variables in one test affects the rest of them.

Note 2
I tried to simulate the Lambda Extensions API locally as well and made really good progress, but I ended up stuck with some tokio threads hanging and not being terminated gracefully when the tests finished. I kept that work on a local branch, may push it in the future if I manage to fix the issue.

Future work

• Add tests for TCP adapter
• Add more scenarios to test API Gateway events.

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[bnusunny]

LGTM