Reimplement MockServer for v2

[fabianfett]

Expected behavior

If the env variable AWS_LAMBDA_RUNTIME_API is not set, we want to spin up the local mock server directly.

Actual behavior

Doesn't work

Steps to reproduce

N/A

If possible, minimal yet complete reproducer code (or URL to code)

No response

What version of this project (swift-aws-lambda-runtime) are you using?

2.0

Swift version

6.0

Amazon Linux 2 docker image version

No response