Add a doc example for using a custom http client

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**
As part of the discussion on https://github.com/apache/arrow-rs-object-store/issues/446 I proposed to @collinoc that they use the custom http client feature to inject a custom header into azure or aws:

https://docs.rs/object_store/latest/object_store/#customizing-http-clients

The basic idea is that you can provide your own wrapper around the lower level http client and inject whatever headers you want

Unfortunately, it seems we are lacking a good doc example

**Describe the solution you'd like**
I would like a clear doc example that for injecting custom headers into an object store using the client

**Describe alternatives you've considered**
N/A

**Additional context**

Start with the object_store crate documentation at the documented “customizing-http-clients” section and inspect how the custom client feature is currently described. Add a clear example showing custom-header injection for an object store, then build or review the generated documentation to confirm the example is complete and readable.

Written by the indexing model from the issue text.