refactor(ckbtc): Clean up types used by ckbtc minter

[ninegua]

We have a few types defined in both ic_btc_interface and ic_sdk, with mostly similar and slightly different implementations. It is confusing to use both of them in the minter, and it is inconvenient because we can't implement From instances directly.

This PR introduces some cleanups as explained below:

1. Define our own Network type (by renaming lifecycle::init::BtcNetwork to crate::Network):

• do not use the one from ic_btc_interface because it is not required anywhere
• do not use the one from ic_cdk directly because it does not implement Display
• convert to the one from ic_cdk because the latter is required to make management calls

2. Define GetUtxosRequest as a type synonym of the one from ic_cdk:

• so that it will be easier to extend with extra fields (not in this PR)
• do not use the one from ic_btc_interface because it is not required anywhere

4. Define our own GetUtxosResponse type:

• do not use the one from ic_btc_interface because it is not required anywhere
• do not use the one from ic_cdk directly because we need Utxo type from ic_btc_interface
• convert from the one from ic_cdk because the latter is returned from management calls

[gregorydemay]

Thanks @ninegua for this clean-up! Some understanding question from my side but otherwise code looks good!

[gregorydemay]

Thanks @ninegua for the changes!