Add SQLite extension entrypoint config to `sqlx.toml`, update SQLite extension example

[supleed2]

Does your PR solve an issue?

Closes #4093

Is this a breaking change?

No, the change is backwards compatible with the existing Vec<String> format.

~~Also, I did not find any tests for parsing the sqlx.toml, please let me know if there are and I just missed them.~~

Same with if you think there needs to be extra documentation / explanations anywhere.

Edit: Failing tests pointed me in the right direction 👌