Wrap the mempool recommended fee rates API

[ShookLyngs]

Recently, the 503 error is often reported from the @rgbpp-sdk/btc when requesting the /api/v1/fees/recommended API from the mempool.space service. Here's one of the failed actions: rgbpp-sdk/action#8802346792. Since the btc-assets-api is planning to replace electrs with the mempool.space API, I suggest also wrapping the recommended fee rates API in the btc-assets-api.

Some benefits:

• Reduce the dependencies used in the @rgbpp-sdk/btc by only requesting from the btc-assets-api
• The btc-assets-api can auto-retry when a 503 error occurs and can cache the result of the recommended fees API

[Flouse]

https://btc-assets-api.testnet.mibao.pro/docs/static/index.html#/Bitcoin/get_bitcoin_v1_fees_recommended