Enhance EmailTask to accept smtp server settings and email_from

[shrmnk]

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?

Replaces #1616 to provide a more customisable EmailTask that defaults to Gmail's servers so that it does not become a breaking change.

Why is this PR important?

EmailTask sounds like a generic name but it was previously hardcoded to Gmail servers.

[marvin-robot]

Here I am, brain the size of a planet and they ask me to welcome you to Prefect.

So, welcome to the community @shrmnk! 🎉 🎉

[cicdw]

This is an excellent enhancement! Thanks for the contribution @shrmnk ! The failing tests will be fixed when we update your branch with #1621

[codecov]

Codecov Report

Merging #1619 into master will decrease coverage by 0.02%.
The diff coverage is 76.92%.