fix: lower Bitcoin testnet3 fee rate by modifying the gas estimator

[ws4charlie]

Describe the Issue
devs are blocked from using Bitcoin testnet3 by high fees. To unblock the devs, we could consider lowering the gas rate by modifying the gas estimator (use more actual fee rate for test purpose).

Expected Outcome
A description of what outcome you are seeking.

[ronaldoguedess]

What needs to be done is to change the depositor fee and use the same fee rate as the transaction, instead of using the average fee rate of the block. Additionally, add support for Testnet4.

This would solve all Bitcoin issues.

[ws4charlie]

@ronaldoguedess I hope this PR can lower the Testnet3 withdraw fee rate and unblock the devs.

[adi44]

This is causing some trouble for us testing on testnet. It would be great if it can be done at priority
Thanks