Give `JsonLinesStream` getters for the inner `AsyncBufRead`

Specifically, give it `get_ref()`, `get_mut()`, `into_inner()`, and `get_pin_mut()` methods.

Problem: `JsonLinesStream` wraps [`tokio::io::Lines`][lines], which does not have a `get_pin_mut()` method to delegate to and whose `into_inner()` method loses data.

[lines]: https://docs.rs/tokio/latest/tokio/io/struct.Lines.html