Allow custom fallback algorithm for bnb #6048

Job	Run time
Check no_std	36s
Check WASM	57s
Build Examples (example_cli)	33s
Build and test (stable, true, --no-default-features)	3m 45s
Build Examples (example_bitcoind_rpc_polling)	35s
Build and test (stable, true, --all-features)	4m 14s
Rust fmt	11s
Build Examples (example_electrum)	53s
Build and test (1.63.0, --no-default-features)	5m 15s
prepare	2s
Build Examples (example_esplora)	57s
Build and test (1.63.0, --all-features)	5m 30s
Build Examples (wallet_electrum)	48s
Build Examples (wallet_esplora_async)	47s
Build Examples (wallet_esplora_blocking)	56s
Build Examples (wallet_rpc)	41s
clippy_check	1m 9s
	27m 49s

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