Remove tokio `rt-multi-thread` feature from `lambda_runtime` and `lambda_extension`, and possibly `rt` as well

Currently the `lambda_runtime` crate has an unconditional dependency on tokio withe `rt-multi-thread` feature enabled:
- https://github.com/awslabs/aws-lambda-rust-runtime/blob/main/lambda-runtime/Cargo.toml#L53
- https://github.com/awslabs/aws-lambda-rust-runtime/blob/main/lambda-extension/Cargo.toml#L35

I'm struggling to see anywhere this dependency is actually used. Meanwhile, given that lambdas only receive one request at a time, it will frequently make sense for callers to only want to use the current thread runtime anyway (ref: #985 ).

I don't think we even need the `rt` (not multithreaded) feature for `lambda-runtime`, not seeing anywhere we are spawning or interacting with feature gated APIs. Though, for #983 we would need it, so probably ok to leave in?

Anyway, is there a reason we need to depend on the multithreaded runtime feature? Am I missing something?