wrap ServerWebExchange to support .getPrincipal()

[Dav1dde]

Fixes #6598

Properly wraps the ServerWebExchange so that in a mocked environment without active SecurityFilterChain the principal can still be received.

[eleftherias]

@Dav1dde please see my comment on the GitHub issue gh-6598.
We do not want to wrap the context in a SecurityContextServerWebExchange, since that does not mimic what is happening when the application is running.
Instead, I suggest creating an HttpBasicMutator, which will include the HTTP basic Authorization header in the request.

[eleftherias]

@Dav1dde Can you confirm that this issue was fixed in gh-7687 and if so please close this PR?

[Dav1dde]

@eleftherias my current issue is resolved with that, but now it's more of a question of how the spring security context interacts with exchange.getPrincipal(). But from my understanding and as we discussed in gh-6598, you (the spring-security team) think the request has to go through a security-filter in order to "get" a principal, which is fair and makes sense - this makes the PR obselete.