Allow 'any' matching strategy for audience claim

[dschanoeh]

The audience matching configuration takes a list of audience values and a token is considered valid when all values are present in the token.
In scenarios where a backend is supposed to serve multiple clients, each having a single but different audience value, a behavior where any match is sufficient is desired. This cannot be achieved with the current configuration option.
As a workaround, one can use the roles matching by setting "roles_key": "aud" but this is not very intuitive (and then no other claims can be checked using this mechanism).
Instead, I propose to add a configuration option similar to scopes_matcher to allow changing of the audience matching strategy.

Also it'd be useful to add to the documentation a note that indicates that the default behavior is to require a match of all audience values.