Consider different approach for optional services from DI

[brockallen]

In a few places we use optional injected services and check for null to adjust behavior. This is a valid approach when using the MS DI system, but we've seen a few requests from customers that use other DI systems and those often don't allow injecting optional/null dependencies. Perhaps we can review our approach and improve it.

[josephdecock]

These are the services that we will need to think about when implementing this. They're the services I found that are resolved with null as a default value in constructors.

• IAuthorizationParametersMessageStore
• IServerSideSessionStore
• ISessionManagementService
• IOperationalStoreNotification
• IEnumerable<IdentityResource>
• IEnumerable<ApiResource>
• IEnumerable<ApiScope>
• IdentityServerLicense
• IDataProtectionProvider
• IServerSideTicketStore
• IdentityServerOptions

[josephdecock]

Pushing this to 8.0, because this would likely be a breaking change for custom implementations that derive from the default implementations.