libsql client: cursor improvements

• Rewrite theCursor introduced in #750 in terms of async-stream lib.
• Replace the futures::Mutex with tokio::sync::Mutex and custom line splitter with tokio-utils LinesCodec. This needs to be validated against Cloudflare workers and possibly other WASM targets.
• See if it's possible to change HttpSend trait to use to make url/auth_token into generic types. This is to reduce copying. However Cloudflare target uses &str in both cases, while hyper uses Uri/HeaderValue. I tried to do it initially, but haven't got enough time to finish it.

Please keep in mind, that projects running in Cloudflare workers are usually build via wrangler, which is not part of the CI pipeline and may potentially be different from calling cargo.

Start by reading the Cursor introduced in #750 and the existing HttpSend trait. Compare the unchecked async-stream rewrite and generic url/auth_token proposal with the completed tokio::sync::Mutex and LinesCodec changes. Done means resolving the remaining checklist items and validating the client for Cloudflare Workers and other WASM targets, including a wrangler build.