Make MultiAuth public API.

[rafalkrupinski]

httpx-auth supports combining auth flows, which is currently handled by implementing SupportsMultiAuth interface. But internally all it does it uses MultiAuth to combine all those Auth instances into one.

This PR exposes MultiAuth, so that any Auth implementation can be directly used as part of this mechanism, e.g. httpx.auth.Digest or any custom Auth classes implemented by users.

[sonarqubecloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

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0.0% Duplication on New Code

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[Colin-b]

hello, we advocate for a single way to create new multi auth. meaning using already exposed SupportMultiAuth as in https://github.com/Colin-b/httpx_auth?tab=readme-ov-file#multiple-authentication-at-once

current implementation is using multiauth class but we would like to keep it internal to change this if needed in the future