Function select_seren_database 

pub async fn select_seren_database() -> Result<String>

Prompts the user to select a SerenDB project and database interactively.

This function will:

1. Get the SerenDB API key (from environment or prompt).
2. Fetch and display a list of projects for the user to select.
3. Fetch the default branch for the selected project.
4. Fetch and display a list of databases for the user to select.
5. Return the connection string for the selected database.

Returns

A Result containing the connection string of the selected database.