Add return types to applicable targets in OidcSecurityService

[JerrettDavis]

Is your feature request related to a problem? Please describe.
There are several methods, fields, and other constructs in OidcSecurityService that do not declare any return types (even any would be sufficient for many). To better represent the intentions of the class, I believe all applicable constructs should have return types added.

Describe the solution you'd like
Return types added to all constructs in OidcSecurityService

Describe alternatives you've considered
None

Additional context
None