Allow custom bucket path

[bribroder]

Presently, anything specified in the sparkctl parameter --upload-to which is not parsed as part of the host component of a URL gets silently dropped and a hard-coded value is used instead:

s3://test.bucket.io/a_prefix_for_my_app is parsed down to test.bucket.io and then /spark-app-dependencies gets added on the end.

This PR allows specifying a separate prefix component at the user's discretion with the --upload-prefix parameter:
--upload-to test.bucket.io --upload-prefix a_prefix_for_my_app

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[bribroder]

@googlebot I signed it!

[liyinan926]

LGTM. Thanks!