Refine subscription strategy for Servlet ExchangeFilterFunctions

[jzheaux]

Related to #7422

There are two Servlet-based ExchangeFilterFunctions in the code base:

ServletBearerExchangeFilterFunction
ServletOAuth2AuthorizedClientExchangeFilterFunction

They both rely on the registration of a CoreSubscriber that populates the Reactor Context with the current Authentication.

The subscriber instances and the actual registrations are different. This means that there are two keys in the Reactor Context, both of which contain the current Authentication when likely only one would suffice.