Failed state inputs

[cicdw]

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Please describe your work and make sure your PR:

• adds new tests (if appropriate)
• updates CHANGELOG.md (if appropriate)
• updates docstrings for any new functions or function arguments, including docs/outline.toml for API reference docs (if appropriate)

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What does this PR change?

This PR adds cached_inputs to all Failed states. When running in Cloud, Prefect will only call the input data's result handlers if all are present, otherwise it will silently pass and not persist the upstream inputs.

Why is this PR important?

This PR stores Failed state upstream inputs if possible, and passes if not. This sets the stage for manual retries in Prefect Cloud. e.g., if my task failed very unexpectedly, I might want to rerun it manually. Prior to this PR, if your task had upstream inputs that would never work. However, with this PR, as long as you set result handlers appropriately this situation will be possible with no additional configuration. cc: @jlowin

NOTE: In implementing this PR I realized that if a user in Cloud does in fact attempt to set a Task Run state that requires inputs, NoResult objects will be passed into all inputs. This can lead to surprising error messages. I attempted to raise an informative error in this situation, but it broke some of our control flow tasks so I decided to table it for now.

[codecov]

Codecov Report

Merging #1557 into master will increase coverage by 0.02%.
The diff coverage is 100%.