Secret result

[cicdw]

Thanks for contributing to Prefect!

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)

Note that your PR will not be reviewed unless all three boxes are checked.

What does this PR change?

Adds a new SecretResult type for handling sensitive data that tasks might return - depends on #2302

Why is this PR important?

PIN 16 work

[codecov]

Codecov Report

Merging #2304 into master will decrease coverage by 0.01%.
The diff coverage is 94.44%.