Add mnemonic_to_descriptors example

[vladimirfomene]

Description

Using bdk-cli it is simple for a new user to generate a mnemonic phrase and descriptors. This might
not be clear for new users when using bdk itself.

Notes to the reviewers

This was initially requested by one user but might be relevant for other users as well.

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[rajarshimaitra]

ACK.. Thanks for writing this.. Would be a really helpful example.. Few comments below..

[danielabrozzoni]

I agree with Raj that this example should also cover creating the actual descriptor, instead of stopping at the xprv

However, you don't really need to manipulate the mnemonic that much to create a descriptor with bdk, as the descriptor! macro also accepts mnemonics (or pairs (mnemonics, passphrase))

    let mnemonic: GeneratedKey<_, Tap> =
        Mnemonic::generate((WordCount::Words12, Language::English))
            .map_err(|_| BDK_Error::Generic("Mnemonic generation error".to_string()))?;
   
    println!("Mnemonic phrase: {}", mnemonic);
    let mnemonic_with_passphrase = (mnemonic, None);

    let external_path = DerivationPath::from_str("m/86h/0h/0h/0").unwrap();
    let internal_path = DerivationPath::from_str("m/86h/0h/0h/1").unwrap();

    let external_descriptor = descriptor!(tr((mnemonic_with_passphrase.clone(), external_path)))?;
    let internal_descriptor = descriptor!(tr((mnemonic_with_passphrase, internal_path)))?;
    println!("{}", external_descriptor);
    println!("{}", internal_descriptor);

This greatly simplifies your whole example, and I think it's the right way to go, as it shows how powerful the descriptor! macro really is :)

[vladimirfomene]

Thanks @danielabrozzoni for helping me with the debugging! @notmandatory this was built on top of #800.

[rajarshimaitra]

tACK 2470305

I will go through the network fix more closely later.. If this is scheduled for 0.25, I think this can go in now..

[notmandatory]

I merged PR #800, go ahead and re-base this PR, just to make sure the history will look good.

[notmandatory]

ACK df905a8