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.

• [x] #259
• [x] https://github.com/spacemeshos/go-spacemesh/issues/5088
• [x] https://github.com/spacemeshos/api/issues/269
• [x] https://github.com/spacemeshos/go-spacemesh/issues/5042
• [x] https://github.com/spacemeshos/go-spacemesh/issues/5131
• [x] https://github.com/spacemeshos/go-spacemesh/issues/5149
• [x] https://github.com/spacemeshos/go-spacemesh/issues/5086
• [x] https://github.com/spacemeshos/go-spacemesh/issues/5089
• [ ] https://github.com/spacemeshos/go-spacemesh/issues/5090

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