Merge bitcoindevkit/bdk-cli#353: fix(log): show warnings by default

3f6b0230ff433c13322dcb54b82f0df51f843df0 docs(changelog): note warnings shown by default (Ikeh Celestine Chukwudi)
33808beef16f9861fa9bf252bcb59affe6d3d981 fix(log): show warnings by default (Ikeh Celestine Chukwudi)

Pull request description:

  Fixes #352

  `env_logger::init()` only shows `error` messages when `RUST_LOG` is unset, so every `warn!` in bdk-cli was hidden from users, including the mainnet notice.

  This PR defaults the log filter to `warn` while keeping `RUST_LOG` as an override (`RUST_LOG=error` hides warnings again), and adds tests for both cases. The default covers all crates as proposed in the issue; limiting it to `bdk_cli=warn` is a one-word change if preferred.

  ### Fixed

  Warnings, including the mainnet notice, are now shown by default without setting `RUST_LOG`.

ACKs for top commit:
  vadim-anfv:
    ACK 3f6b0230ff433c13322dcb54b82f0df51f843df0
  tvpeter:
    ACK 3f6b0230ff433c13322dcb54b82f0df51f843df0

Tree-SHA512: ad39ee1942f01e2e043462b866299e594e07891c7fdb85d9b85eb8ae1f6f17b1f0b410c81a7f5b71082bb6824a920e57b18fb49bf689f05225c17eb25add163e