fakewallet values in config

[Jeezman]

Description

when runing cargo run --bin cdk-mintd with fakewallet set in config.toml it gives the error

    Finished `dev` profile [unoptimized + debuginfo] target(s) in 11.16s
     Running `target/debug/cdk-mintd`
thread 'main' panicked at crates/cdk-mintd/src/main.rs:214:60:
Fake wallet defined
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

This PR

• adds the defaults for fakewallet to the sample config.toml file
• adds custom error output to assertion to assertion

assertion before

thread 'main' panicked at crates/cdk-mintd/src/config.rs:227:38:
assertion failed: settings.fake_wallet.is_some()
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

assertion after

thread 'main' panicked at crates/cdk-mintd/src/config.rs:219:38:
FakeWallet backend requires a valid config.
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Notes to the reviewers

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Suggested CHANGELOG Updates

CHANGED

ADDED

REMOVED

FIXED

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Checklist

• [x] I followed the code style guidelines
• [x] I ran just final-check before committing