Move `block_size` KWarg to initialization?

I'm browsing through the backends, and it looks kind of inconsistent how we sometimes hand `block_size` or `block_samples` to `read_data`, and sometimes we pass it to the initialization?

Furthermore, `def audio_open(path):` doesn't take a `block_size` parameter, so users can't use it unless they use a backend directly.

I think we could improve the lib by adding `block_size` to `audio_open`. For the existing backends that have `read_data(blocksize=*)` we could still allow them to override blocksize on the read_data level. It shouldn't be breaking change if we do it this way.

Start by tracing audio_open(path) and the backend initialization and read_data(blocksize=*) entry points to compare how block_size and block_samples are currently passed. The change is done when audio_open accepts block_size, existing read-time overrides remain possible, and the behavior is consistent across the affected backends.

Written by the indexing model from the issue text.