feat: Add bearer token http middleware [3/4]

[jcscottiii]

This middleware adds support for Bearer token authentication in your API. It extracts the Bearer token from the Authorization header and passes it to an Authenticator interface for verification. The implementation of the Authenticator interface is provided separately.

Key features

• Extracts Bearer token: The middleware extracts the Bearer token from the Authorization header if present.
• Passes token to authenticator: It passes the extracted token to an Authenticator interface, which is responsible for verifying the token and returning a user object if the authentication is successful.
• Handles missing or invalid tokens: The middleware handles cases where the Authorization header is missing or malformed by returning appropriate error responses.
• Customizable error responses: The errorFn parameter allows customization of the error response returned when authentication fails.

How to use it

1. Implement the Authenticator interface to define how the Bearer token is verified and a user object is returned.
2. Create an instance of the BearerTokenAuthenticationMiddleware using your Authenticator implementation and an errorFn to handle authentication errors.
3. Apply the middleware to your API routes.

Additional resources

• OpenAPI specification: https://swagger.io/docs/specification/v3_0/authentication/bearer-authentication/
• Google Cloud Endpoints documentation: https://cloud.google.com/endpoints/docs/openapi/authenticating-users-custom

Important notes

• This middleware assumes that the route requires authentication if the BearerAuthScopes field is set in the request context.
• You are responsible for setting the ctxKey in the request context when authentication is needed.

This is a split up of #1118