Add support for FileDSNs

[AetherBreaker]

No description provided.

[gordthompson]

A File DSN works fine for me.

import asyncio

import aioodbc


async def async_main():
    cnxn = await aioodbc.connect(
        dsn="FILEDSN=/__tmp/my_file_dsn.dsn;UID=scott;PWD=tiger^5HHH",
    )
    crsr = await cnxn.cursor()
    await crsr.execute("SELECT 1 AS foo")
    foo = await crsr.fetchval()
    print(foo)  # 1
    await crsr.close()
    await cnxn.close()


asyncio.run(async_main())