Feature: Add Basic Authentication Support

[AH-dark]

Summary

This PR adds support for HTTP Basic Authentication in the ClientBuilder and Client structures of the dxr_client crate.

It allows users to connect to XML-RPC endpoints that require a username and password, without manually customizing the underlying reqwest::Client.

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What's Changed

• Added username and password fields to ClientBuilder and Client.
• Introduced a builder method:

  pub fn basic_auth(mut self, username: String, password: String) -> Self

• Updated Client::with_client to accept optional authentication credentials.
• In Client::call, added logic to apply .basic_auth(...) to reqwest::RequestBuilder when credentials are provided.

Motivation

Some enterprise or internal XML-RPC APIs are protected with Basic HTTP Authentication. This patch provides first-class support to enable seamless integration with such systems.

[decathorpe]

Thanks for your PR!

I'm not sure if I actually want the builtin reqwest client to grow more featureful. Both the reqwest client and the axum server are intended to be minimal "getting started" implementations. Though I suppose this is a very simple feature to add since it just adds a few lines of code ...

There are some potential issues with the code that I see, I'll leave those as inline reviews.

[decathorpe]

Thank you, looks good to me now!

Do you need a new release with these changes?