Validate the signer field in the JWT header

[sankichi92]

From https://docs.aws.amazon.com/elasticloadbalancing/latest/application/listener-authenticate-users.html:

To ensure security, you must verify the signature before doing any authorization based on the claims and validate that the signer field in the JWT header contains the expected Application Load Balancer ARN.

This PR implements this validation for the signer field in the JWT header.

[satoyuichi]

LGTM