Add EnvVarSecret

[jlowin]

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?

This PR adds a new Secret class that pulls secret values from arbitrary environment variables. The values are treated as sensitive, due to the Secret subclass, but can be passed to any downstream task for processing / use. In addition, values can be cast to any appropriate type.

Why is this PR important?

[codecov]

Codecov Report

Merging #1683 into master will increase coverage by 0.01%.
The diff coverage is 100%.

[cicdw]

Ultimately I love this - it expands our secrets library and shows people how to work with their own custom secrets. I did make a lot of requests for documentation changes though

[cicdw]

two minor typo-things, this LGTM otherwise!

[jlowin]

Thanks @cicdw

[cicdw]

@jlowin changelog conflicts