server/btc: use external fee rates

[buck54321]

Replaces #2638, which was never merged to the release branch
Steps on #2769. Does not replace that work, but offers some patterns that I think are useful and should be adapted to Tatanka.

Enables external fee rate estimation for Bitcoin. estimatesmartfee is not always the best estimate and requires priming. We retain estimatesmartfee as a backup.

• Sources have rank / priority
• Sources have customizable refresh times
• Registered APIs for Tatum and Blockdaemon are available via a new configuration file format option
• A composite score is calculated based on the the valid scores from the best available priority group, with the weight of the individual score in the composite falling off over time after a period of full validity