Add filer.Filer to read notebooks from WSFS without omitting their …

…extension (#1457)

## Changes
This PR adds a filer that'll allow us to read notebooks from the WSFS
using their full paths (with the extension included). The filer relies
on the existing workspace filer (and consequently the workspace
import/export/list APIs).

Using this filer along with a virtual filesystem layer
(https://github.com/databricks/cli/pull/1452/files) will allow us to use
our custom implementation (which preserves the notebook extensions)
rather than the default mount available via DBR when the CLI is run from
DBR.

## Tests
Integration tests.

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Co-authored-by: Pieter Noordhuis <pieter.noordhuis@databricks.com>