Implement database streaming for get subscribers endpoint

[chris13524]

Right now the full list is pulled into memory and then streamed out as JSON. Implement a lazy pulling mechanism so that memory usage doesn't spike when this endpoint is called.

[chris13524]

Not as easy as I thought it was going to be. Some reading material:

• How to stream a query from sqlx using axum tokio-rs/axum#400
• Upload and download with Axum, streaming tokio-rs/axum#1638
• How to respond streaming data from Receiver? tokio-rs/axum#397
• https://docs.rs/axum-streams/

Serialize to stream as JSON: serde-rs/json#345 (comment)

An attempt is on the branch: https://github.com/WalletConnect/notify-server/tree/308-implement-database-streaming-for-get-subscribers-endpoint