Support (+) on JOINED_FILTERS

It would be handy to also support (+) on JOINED_FILTERS, e.g. for BasicAuth on a url-pattern, otherwise you need to state the whole default filter chain, or need to add the new filter using SpringSecurityUtils.clientRegisterFilter.

(+) should allow to add to the start or tail of the list, depending on its position relatively to JOINED_FILTERS.

e.g.:

[pattern:'/api/**', filters: '+myFilter, JOINED_FILTERS, -securityContextPersistenceFilter, -exceptionTranslationFilter'],

In this example, a myFilter is added to the head of the list.

Start by tracing how JOINED_FILTERS and SpringSecurityUtils.clientRegisterFilter handle filter lists, using the example in the issue as the target behavior. Verify that a filter prefixed with (+) is placed at the head or tail according to its position relative to JOINED_FILTERS, while the existing removals still work.

Written by the indexing model from the issue text.