supporting multiple Custom Domains

[chewmanfoo]

Please update jwksClient and jwt.verify such that it supports a collection of domains, rather than just one. With support for only one domain, the implementation of multiple Custom Domains creates a knife-edge cutover deployment. However, since the Auth0 API supports the custom domain header in mgmt API requests whether multiple custom domains are used or not, it seems everywhere else in Auth0, multiple custom domains are supported. This allows for an easier implementation of mutiple custom domains in Auth0. However, since the lambda-auth0-authorizer doesn't support multiple domains in the jwksClient and the jwt.verify function, the authorizer has to be updated when a new custom domain is implemented. Further, the node.js code gets unnecessarily complex if multiple domains aren't supported here.