Allow Custom File Systems To Override The Walk Method

[hamzajzmati]

Motivation for features / changes

When registering a file system, we would like to be to use our custom walk method. This is useful for using multi-threading in the walk and other file system specifics.

Technical description of changes

Instead of using the gfile implementation of walk, it will check first if the file system has its own implementation.

Detailed steps to verify changes work correctly (as executed by you)

Tested locally with a custom file system. Used custom walk method when it existed. Used old logic when file system didn't have an override.

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