Make filters more easily composable

Right now, it's somewhat difficult to compose filters so that some filters are only applied if one or more conditions are met. A usual example of this is that some filter only needs to apply to a subset of clients and most filters only support one user setting that's used to match whether the filter is active or not. For this particular example, converting the setting into a regular expression would be an easy way to solve it but it is both error prone and not easy to combine with e.g. hostname based filtering.

An approach similar to that of the PAM authentication modules would fit the current modular architecture of MaxScale. Providing simple, composable modules would allow users to configure the filters in a manner where they're applied precisely to the set of users that need them.