feat: allow to define a function to select credentials

[qoomon]

The author should do the following, if applicable

• Add tests
• Run tests
• bun run format:fix && bun run lint:fix to format the code
• Add TSDoc/JSDoc to document the code

In my project the credentials can be either in Authorization or in X-Authorization header, therefore I added an optional function to allow arbitrary logic to gather credentials from the request.