Spring-security: Missing Reactive SwitchUserWebFilter for user impersonation

[aotrzonsek]

Expected Behavior

Add the possibility to impersonate a user in Reactive applications as it is possible by using the SwitchUserFilter.

Current Behavior

It is not possible to switch the user context, at least I couldn't find anything equivalent.

Context

It’s a common use-case that some admin users should be able to log in as any other user.
In our applications we have used this feature for customer support analysis where the analyst can access the system as the real user.

[rwinch]

Thanks for the report @aotrzonsek Would you be interested in submitting a pull request?

[aotrzonsek]

Sure why not, I have to implement it anyway. I will add a comment after I have finished the implementation.

[aotrzonsek]

All issues found during the code review have been removed.