Backport of IBM#2339

This is necessary to be able to migrate from an assignment strategy to another: it lets the consumers advertise a list of supported strategy so the group can choose one that is supported by all members.

A migration can be done as follows:

• all members support the old strategy only
• members are rolled out to add support for new strategy while keeping the old one
• during the rollout the group picks the old strategy as it is the only one supported by every member
• after the rollout, all members supports both strategy. The one chosen is based on the order in the config