fix: Ensure authorized span headers are injected as str

[Wesitos]

What does this PR do?

It injects the authorizer span headers as a regular string instead of a byte string.

Motivation

The Python 3.7 lambda runtime can't serialize responses that contain byte strings.

Testing Guidelines

Additional Notes

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

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• This PR contains breaking changes that are documented in the description
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[joeyzhao2018]

Putting request changes here to block. We had to use encoding because

Injecting extra data to the original response can break invocations using cached authorization data. Symptoms: The first (authorizing) invocation succeeded. But the following requests received Internal Server Error messages.

Therefore, TODO on our side: we need to test the encode and decode case here. We should add the case to the the integration test.

Hi @Wesitos, could you please open an issue on the serialization problem mentioned in the description here so that we can have more information? Thank you in advance.