[source-file] add fixed width file support

[mjgatz]

Tell us about the problem you're trying to solve
We want to add fixed width file support to the file source

Describe the solution you’d like

It appears that the source file connector is using pandas reader as the driver, pandas has a fixed-width reader:
read_fwf

Happy to contribute here
Thanks!

[amirsharapov]

This would be a really useful feature for us at our company as we deal with fixed width files.