NIFI-6175: Improving Livy support

[mattyb149]

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Description of PR

This PR picks up where #3511 left off and adds some handling for things like sessions that die immediately on startup. Also there were "file exists" validators on filenames that only have to exist on the target system not the NiFi system, so I changed those as well. I tried on both Livy for Spark 1/2 and Livy for Spark 3 using the Livy 0.7.0 API.

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[exceptionfactory]

@mattyb149 PR #5620 includes updates to several tests, replacing the custom test Jetty Server with the OkHttp MockWebServer. This approach makes it much easier to test components that need HTTP client access. Additional changes in that PR make use of mocked services, as opposed to actual Controller Service implementation, so that might be a helpful starting point for some of the improvements in this PR.

[github-actions]

We're marking this PR as stale due to lack of updates in the past few months. If after another couple of weeks the stale label has not been removed this PR will be closed. This stale marker and eventual auto close does not indicate a judgement of the PR just lack of reviewer bandwidth and helps us keep the PR queue more manageable. If you would like this PR re-opened you can do so and a committer can remove the stale tag. Or you can open a new PR. Try to help review other PRs to increase PR review bandwidth which in turn helps yours.