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?

lambda-runtime/Cargo.toml と lambda-extension/Cargo.toml の tokio 依存関係の宣言を確認し、続いてこれらの crate で rt および rt-multi-thread features によって有効になる API を検索します。いずれかの feature が必要かどうかを確認し、サポートされている runtime のユースケースに対して依存関係の設定が引き続きビルドできることを検証します。不要な features が削除されるか、その必要性が文書化されれば issue は解決です。

索引モデルが issue の本文から書いたものです。