Simplify Multitenancy Sample

[jzheaux]

With spring-projects/spring-boot#19426, the multi-tenancy sample can be simplified, allowing Boot to construct the JwtDecoder and OpaqueTokenIntrospector.

While here, this sample can also be simplified in a few other ways:

• Use a header instead of a path element
• Extract the custom AuthenticationManagerResolver into its own class
• Remove the use of the word "tenant" in places where it's implied