Create @ConditionalOnMissingServletFilter

[bclozel]

This Conditional annotation should:

• look for a bean of a particular type
• if it doesn't find one, look for a FilterRegistrationBean that will register a Filter of a particular type

This condition should improve the auto-configuration of Servlet Filters.

We could go for a shorter name @ConditionalOnMissingFilter, but this could clash with a future similar annotation for Spring Web Reactive WebFilters.