Interpolate fee estimates to allow for arbitrary targets

[tnull]

As of 0.28.0, BDK's estimate_fee is just handing through individual fee estimates of the underlying Blockchain implementation.

For example in case of the Esplora client, it would just fall back to 1 sat/vb if an unknown target (i.e., not in {1-25, 144, 504, 1008}) is given:

https://github.com/bitcoindevkit/rust-esplora-client/blob/615f76f4256fd8abe95ce76b1fd0d8cf7f81a51a/src/lib.rs#L83-L97

It would be nice to interpolate from the given values rather than silently defaulting to 1sat/vb. If we don't want to interpolate for one reason or another the valid target values should probably be defined as part of a ConfirmationTarget enum, and this behavior should clearly be documented, as it isn't currently.

I can't find estimate_fee in the current master, so not sure what the plans are for BDK 1.0.

[tnull]

Closing as BDK 1.0 won't offer a fee estimation API anymore.