Modular node architecture

[lrettig]

Meta issue to track several workstreams related to making the current node architecture more modular. Much of this work involves updating go-spacemesh to manage multiple smesher identities.

• Post as a service #259
• beacon: support multiple smeshers go-spacemesh#5088
• Draft API for Proof generation api#269
• Integrate PoST draft API into node go-spacemesh#5042
• gRPC: add mTLS authentication to post service go-spacemesh#5131
• activation: support multiple smeshers go-spacemesh#5149
• cert: support multiple smeshers go-spacemesh#5086
• configuration to load multiple smeshers from disk go-spacemesh#5089
• dynamic reconfiguration of smeshers used in the node go-spacemesh#5090

See also: https://github.com/spacemeshos/go-spacemesh/labels/feat%2Fmulti%20smeshers