[bisq-network/bisq] Allow refundagents to set their own fees for payouts. (#5158)

[refund-agent2]

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### Description

Bisq takes the recommended fee rate for payouts, which is too volatile for refund agent needs. I wait for days with low fees but as I need a few minutes to prepare the payout, it jumps from 10 or 15 sats to over 50 sats/vB and I have to wait until a block is mined.

#### Version

1.5.5

### Steps to reproduce

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Make a payout with withdrawal transaction fee set to 12. Bisq will use the recommended fee rate, displayed at the bottom left.

### Expected behaviour

<!--description of the expected behavior -->
Use the witdrawal tx fee or let me set it when creating the payout. I will respect the 10 sat min set by the software for trading transactions and keep an eye for stuck transactions.

### Screenshots
About 10 minutes ago, recommended fee was 10 sats/vbyte.

![imagen](https://user-images.githubusercontent.com/75325927/107112951-3e327300-6853-11eb-9282-e5cc55adfda4.png)

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https://github.com/bisq-network/bisq/issues/5158
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