Refactor main.rs into a new module generic in its client (#724)

* Move entire main.rs into cli.rs

This is the smallest change to make things still compile. Refactoring
comes in the next commit.

Signed-off-by: James Bornholt <[email protected]>

* Refactor to make `main` generic in the client

This will let us run the whole mount-s3 binary with different client
implementations

Signed-off-by: James Bornholt <[email protected]>

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Signed-off-by: James Bornholt <[email protected]>