Add else option to the RequestMatcherPlugin

[sagikazarmark]

Q	A
Bug?	no
New Feature?	yes
Version	1.4

Actual Behavior

The request matcher checks if the given request matches the requirements, and executes the delegated plugin. In cases where we want to use an if-else logic to decide between two plugins this does not work well, and currently requires two RequestMatcher plugins.

Possible Solutions

Provide a second plugin to the RequestMatcherPlugin which gets called when the request is not a match.

It would also probably make sense to rename the RequestMatcher plugin to Conditional (this would be aligned with the Conditional authenticator in the message repo)

[dbu]

do we want to do this renaming, or do we drop that idea?