Multiple Header Key and Value Support

[cgetzen]

Problem

We would like to migrate from static credentials to JWTs, solely using the "Authorization" header.

Solution

Static credentials support multiple header values being present. For example, the header "Authorization: invalid, valid, moreinvalid" can be configured to pass.

traefik-jwt-plugin should support the same pattern.

Alternatively, If this plugin supported checking duplicate header names instead of just the first one, it would allow transition from static keys to JWTs. For example, the request curl -H "Header: ${invalidJWT}" -H "Header: ${validJWT}" example.com would need to be accepted.