[bisq-network/bisq] MempoolFeeRateProvider should use onion for api query (#5749)

[unofficial bisq contributor]

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### Description
We have secure onion communication built inside, so don't leave the tor network and put load on exit nodes where possible



#### Version

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### Steps to reproduce

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### Expected behaviour
The fee estimation should use http://mempoolhqx4isw62xs7abwphsq7ldayuidyx2v2oethdhhj6mlo2r6ad.onion/api/v1/fees/recommended as recommended by code example of http://mempoolhqx4isw62xs7abwphsq7ldayuidyx2v2oethdhhj6mlo2r6ad.onion/api


### Actual behaviour

 instead of https://" + "mempool.space" + "/api/v1/fees/recommended


### Screenshots

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#### Device or machine

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#### Additional info

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https://github.com/bisq-network/bisq/issues/5749
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