Implement the option for having a list of access-checkers instead of just one.

[bashir2]

Currently we can only have one access-checker; to combine multiple access-checkers one should implement a new plugin. It is probably better to let multiple access-checkers be provided and they will be checked in the order provided. For example, a setup might be ACCESS_CHECKER=patient,list,backend where a provided Bearer token is first checked against the SMAR-on-FHIR patient access-checker. If it fails the check, then it will be checked against the ListAccessChecker, etc.