Make defensive collection copy as Collections.unmodifiableCollection
does not protect from the source collection direct modification.
Use Mono#map instead of Mono#flatMap as it allocates less.
Use less operators to reduce allocations.
Use lambda parameter instead of outer method parameter
in authenticationManagers#computeIfAbsent()
to make it non capturing so it could be cached by JVM.
Propagate cause for InvalidBearerTokenException.