Add raw pre_send method

[KodrAus]

For #281

I've marked this as a breaking change because it depends on some breaking work done already.

Adds a pre_send_raw method to the Sync and Async clients that can be used to inspect and tweak a raw request before it goes out. This makes it convenient to do things like sign ready-to-send requests, but shouldn't be used for cases like adding simple headers or query params that should be done through params.

I'll add an example case and this should be ready to go.

Changes

elastic_requests

• Rename HttpRequest to Endpoint
• Rename Url to UrlPath
• Remove HttpMethod and re-export http::Method

elastic_responses

• Export http::StatusCode
• Replace case taking a u16 status with StatusCode

elastic

• Add some new raw http request types that include headers and can be inspected
• Allow tweaking a raw request before sending

TODO

• Replace u16 in elastic_responses with http::StatusCode (this is a bit of churn but shouldn't impact people much)
• Make the body types readable with streams again (means buffering files which should be called out in the docs)
• Make sure the module hierarchy makes sense

Essentially I want to get a foot in the door using the sharable types from http.

[KodrAus]

Looks like we'll need a dedicated request type for this to work, because you can't read reqwest request bodies so the usecase of signing requests won't work.

I think it's a good chance to pull in the http crate and use its types.

[KodrAus]

Ok I'm going halfway with this, renaming types in elastic_requests the their intent is clearer (changing HttpRequest to Endpoint) and adding an inspectible http request type to elastic. It needs a lot of cleanup but the basics are there.

I think I need to shift the requests and sender modules around a bit. It's got a lot of stuff sitting in the respective roots.