Supporting multiple client processes with the same client cert

[msheller]

Is your feature request related to a problem? Please describe.
In some cases, a single client identity may want to run multiple clients, e.g. a site has collected multiple datasets but is under contract such that they cannot mix datasets within a single process. In such cases, the same identity will need to run multiple openfl client processes. Another example is scientific research, where a single site wants to simulate multiple sites without the onerous process of maintaining multiple identities (i.e. certs).

Describe the solution you'd like
A collaborator process should be able to identify itself as a (user id, dataset) pair, and the aggregator can track each of these separately. All such pairs are known a-priori, so the list of all client processes can be static.