Create /extended/v2/mempool/fee-rates endpoint

[aryzing]

It's great that we have the /extended/v2/mempool/fees endpoint to get a sense for what a reasonable transaction fee should be.

However, transaction sizes may vary. It would be great if we could have a similar endpoint, e.g. /extended/v2/mempool/fee-rates which returned average lom/mid/high fee rates for the different transaction types.

[smcclellan]

@aryzing Thank you for filing this. Could you share more about how you might use this?

[aryzing]

The two use-cases would be,

1. Displaying in an app's UI
2. Setting the transaction fee based on the size of a transaction

The assumption here is that a tx's size matters to miners when considering whether to include it in a block based on the fee. Does it matter though? Are transactions with larger fee rates selected first to be included in a block, or is the total fee amount the only number that matters?