Support reading the gateway key from the Authorization header

[jtpio]

Description

As mentioned in https://mozilla-ai.github.io/any-llm/gateway/quickstart/, the X-AnyLLM-Key header must be provided to specify the master key when using the any-llm gateway:

  -H "X-AnyLLM-Key: Bearer ${GATEWAY_MASTER_KEY}" \

However it may be impractical to provide with existing generic OpenAI providers.

So it would make sense to also support reading the key from the Authorization header.

PR Type

🆕 New Feature

Relevant issues

Checklist

• I have added unit tests that prove my fix/feature works
• New and existing tests pass locally
• Documentation was updated where necessary
• I have read and followed the contribution guidelines```

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
see 43 files with indirect coverage changes

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[jtpio]

The last commit should have addressed the lint issues.

Marking as ready so folks could take a first look already.

[daavoo]

Thanks @jtpio ! Changes make sense to me.
I have tested manually and it works.

Will add a test actually using the OpenAI client to do a completions request in a followup