Add Verbose Agent Logging Option

[joshmeek]

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• adds new tests (if appropriate)
• updates CHANGELOG.md (if appropriate)
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What does this PR change?

Closes #1501
Adds new agent log level config option
prefect agent start --verbose/-v to enable or config.cloud.agent.level

Why is this PR important?

Allows for finer grained logging from the agent

[codecov]

Codecov Report

Merging #1532 into master will increase coverage by <.01%.
The diff coverage is 91.3%.

[cicdw]

Some requests for variations on a few logs - this is going to be great!

[cicdw]

🚀 excellent 🚀

[jlowin]

Not sure if it matters but this is not a Prefect logger

[cicdw]

That's intentional - these logs are intended to be streamed to stdout in whatever platform the agent is running in. We should introduce the notion of "Agent logs" to store in Cloud, and find a way of exposing them in a way that is flow-independent but that's a future feature.