feat: use custom auth header name

[amyrprv]

Description

This PR introduces a change that allows the authorization header to be configurable, enabling the use of a custom header name for authorization. Previously, APISIX used a fixed authorization header, but with this update, users can define their own header name in the configuration. This enhancement offers greater flexibility for integrating with systems that require non-standard authorization headers.

Fixes # (issue)

Checklist

• [x] I have explained the need for this PR and the problem it solves
• [x] I have explained the changes or the new features added to this PR
• [x] I have added tests corresponding to this change
• [x] I have updated the documentation to reflect this change
• [x] I have verified that this change is backward compatible