Add support for HTTP requests intervention (i.e. support for AWS Elasticsearch)

[Veetaha]

Hi, this is a splendid proxy you made!

I'd be grateful if we could use it to issue requests to AWS hosted Elasticsearch cluster. This means there should be an API that enables egress HTTP requests modification before sending because AWS requires them to be signed according to signature v4 algorithm.

For example, the other elasticsearch-rs crate already introduced such an API in this PR and which shows one option to implement the intervention.

Or I can propose a more familiar to me TypeScript client's approach that accepts a Connection class as a config parameter and which could be translated into Rust trait.

Hope to hear from any contributor!