Add info about suggested txn fee for BTC/ETH withdrawal

[gcharang]

Right now, a user is blind to the txn fee market while using the app, and only a very advanced user would have the knowledge and ability to estimate the required fee and set it using the custom fee option.

An interface like the one in a BTC electrum wallet might be good to integrate.

Example:

The slider gives estimated blocks before confirmation and moving it also changes the custom fee input.

If the above can't be done, just reporting the txn size and a link to an external fee estimator site like https://bitcoinfees.earn.com/ should be good.

The site also has an API we can use to display the recommended fee in App https://bitcoinfees.earn.com/api

[cipig]

the output of the method withdraw in mm2 is showing the total fees that will be used, so \"method\":\"withdraw\",\"coin\":\"BTC\",\"to\":\"xxxxxxxxxxxxx\",\"max\":true,\"fee\":{\"type\":\"UtxoPerKbyte\",\"amount\":0.00004}} (with custom fee of 4 sats)
shows

   "fee_details" : {
      "amount" : "0.00001952"
   }

if no fee is passed to withdraw, mm2 uses the fee from estimatesmartfee call of bitcoind