Aggregator address and port should not be part of the plan

[DL8]

Is your feature request related to a problem? Please describe.

Currently in aggregator-collaborators architecture, the aggregator address and port are defined in the plan. In a serverless environment, this makes it impossible to start a federation: the plan file is defined as trusted in Gramine manifest and therefore cannot be modified. Consequently, since the aggregator host is not known a-priori, it's impossible to build and launch the aggregator properly

Describe the solution you'd like

Aggregator address and port should not be part of the plan, but passed as an argument instead:

1. In fx aggregator start: will listen on the provided port (a default value should be defined) on the provided network interface (default 0.0.0.0, i.e. all)
2. In fx collaborator start: will connect to the provided host and port (the same default port as aggregator should be defined)
3. Server key and certificate filenames (in cert/server) should not include the hostname (analogous to cert/cert_chain.crt)