feat(rpc): add ability to estimate fee

[theborakompanioni]

It turned out that one of the biggest problems for users is that they cannot estimate fees, hence not knowing how much "using the software" will cost them. This is not a big deal when using the CLI, as enough info is logged, but certainly an issue when using the RPC API.

It would be nice to provide more insight to web users. Would it be possible to add a way to provide something like "Estimated miner/tx fee for each cj participant" (like in the CLI) via the RPC API?

e.g. request:

{
  "txfee": 3,
  "tx_fees_factor": 0.2,
  "max_cj_fee_abs": 10000,
  "max_cj_fee_rel": 0.0003,
  "max_sweep_fee_change": 0.8,
  "amount": 0, // sweep
  "mixdepth": 1,
  "counterparties": 9,
}

response:

{
   "estimated_mining_fee": 341000,
   "estimated_collaborator_fee": 15410,
}

This is just an example and should just demonstrate what is trying to be achieved.
What do you think?

[editwentyone]

bump :) that would be so awesome