Expose several private functions to allow easier implemenation of `HttpService` and `HttpConnector`

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**

Hi there 👋 I've been trying to implement the `HttpService` and `HttpConnector` traits to allow me to trace the HTTP calls that object store is making. However, several functions used in the implementation for `HttpService` on `reqwest::Client` (which I am essentially replicating, with the addition of the tracing) are private so I cannot reuse them in my implementation.

These are:
* `HttpError::reqwest`
* `HttpRequestBody::into_reqwest`
* `‎ClientOptions::client`

**Describe the solution you'd like**
These functions to be made `pub`.

**Describe alternatives you've considered**
None, however, keen to hear if the there would be a better way to trace the calls.

**Additional context**
N/A

Happy to handle the PR for this (should be trivial), just wanted to make sure it was not misguided!

Start by locating the definitions of HttpError::reqwest, HttpRequestBody::into_reqwest, and ClientOptions::client, then read the HttpService and HttpConnector implementations that need them. Check the crate's visibility and API conventions before making the functions public; done means those implementations can reuse the functions without duplicating them and the existing project checks pass.

Written by the indexing model from the issue text.