Allow SQLiteConnection creation without specifying an event loop

[gwynne]

[The equivalent of] cherry-picked from vapor/postgres-nio#388:

SwiftNIO has landed EventLoopGroup and NIOThreadPool singletons in 2.58.0. See apple/swift-nio#2471.

We should add an additional SQLiteConnection open method, that internally uses the existing one, but uses the NIO singletons:

extension SQLiteConnection {
    static func open(
        storage: Storage = .memory,
        logger: Logger = .init(label: "codes.vapor.sqlite")
    ) -> EventLoopFuture<SQLiteConnection> {
        Self.open(
            storage: storage,
            threadPool: NIOThreadPool.singleton,
            logger: logger,
            on: MultiThreadedEventLoopGroup.singleton.any()
        )
    }
}

• Add API
• Remove all EventLoopGroup usages in the README