Extension loading support

[penberg]

The libsql Rust crate supports extension loading. Let's add support for that in the Go driver, similar to:

https://pkg.go.dev/github.com/mattn/go-sqlite3#SQLiteConn.LoadExtension

[ashmortar]

any word on this? I'd like to use uuidv4() in file dbs in tests, but I can't seem to find a good way to load extensions in to get parity with turso-cloud.