Support for multiple mesh services

[yorugac]

Initial effort to add support for mesh services was done by Hans Knecht: first by adding support for Istio, then by working on #73
This last PR was never finished: it requires quite a lot of work, and nobody had the time for it so far.

The PR #73 was written and reviewed with Istio + LinkerD in mind, but it makes sense to briefly review other mesh services solutions, to understand better what "generalized" support for mesh services would look like in the operator. Additionally, it'd be good to have some additional feedback from community on this feature request.