Return estimated message fee via API

[tboeckmann]

Is your feature request related to a problem? Please describe.
In order to show users the estimated fee to send a message, we need an endpoint that returns the current estimated fees in the context of a protocol, network, and envelope type.

It should take into account the cost of the on-chain data and sending the transaction with medium priority.

Describe the solution you'd like
GET /messages/fee?protocol=ethereum&network=ropsten&envelope_type=0x01 returns:

{
  "fee": "167248000000000",
  "unit": "wei"
}

Ethereum
See tools here
Etherscan may be the best place to get data from as this is already used for the tx index and is considered widely accurate.

Edgeware
E.g. see substrate fees here

Other considerations
Envelope has an impact? It might be handled differently than as a param- I'm open to suggestions.

[sa3664]

@tboeckmann Could you please assign this issue to me .