Make sqlx optional for file-store

[michaeldjeffrey]

sqlx found it's way into the main compilation path of file-store, even though it was marked as optional.

This PR follows the pattern in helium-crypto-rs is hiding all sqlx related impls behind #[cfg(feature = 'sqlx-postgres')].

The sqlx-postgres feature is default enabled for file-store.
To test this, making a sample rust project with a file-store dependency will show that sqlx is not included unless it's asked for.

[dependencies]
file-store = { path = "../oracles/file_store", default-features = false }

> sqlx-test git:(master) ✗ cargo tree -i sqlx                 
error: package ID specification `sqlx` did not match any packages