Support multiple certificate types

Problem/Motivation

Currently the module ships with only one certificate type, fillpdf, and architecturally doesn't support multiple certificate types in use on one site.

Modules could implement a different certificate type, copying the logic in the fillpdf implementation, and that would allow a different type to be used. However, it wouldn't allow two different types to be used at the same time because the controller will conflcit.

Steps to reproduce

Proposed resolution

Implement a plugin or tagged service where each certificate type checks the entity type of the DER reference, and provides the Response object back.

Move the controller to the main lms_certificate module, and have this call the plugins/tagged services.

Remaining tasks

User interface changes

API changes

Data model changes