Introducing: Environment Callbacks

[joshmeek]

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 is an implementation of PIN-12 for environment on_start and on_exit callbacks. It also adds a short concept doc for Environments and Storage.
Closes #1495
Closes #1574
Closes #1581

Why is this PR important?

Environment related callbacks are important tools for performing work outside of environment infrastructure and flow operations.

[codecov]

Codecov Report

Merging #1610 into master will increase coverage by 0.01%.
The diff coverage is 100%.

[cicdw]

This is super cool / so simple. A few comments / questions on first pass

[cicdw]

Excellent! I'm excited to see how this gets used.