Add support for event subscribers

Problem/Motivation

Having a suitable future replacement for the global hook system in mind, Drupal 8 introduced support for object-oriented Symfony events. A suggested event for altering the URL options during authentication in the issue queue brings up missing event support in the entire OpenID Connect module.

While preparing for a stable first release of the OpenID Connect module, we should add event support as well. This would allow third-party module developers to decide whether to use hooks or events for extending/altering the data of the OpenID Connect module.

Proposed resolution

Provide an according event for every hook of the OpenID Connect module.

Remaining tasks

Create a generic and easily extendable approach to cover all hooks with events providing the same information as the newly added $context parameter of our hooks.

User interface changes

None.

API changes

Additional events allow to subscribe for altering data or reacting at various stages of the OpenID Connect configuration/authorization process.

Data model changes

None.