Add PrefectFormatter to reduce logging configuration duplication

[zanieb]

Previously, flow run and task run loggers required their own formatters and did not propagate to the prefect logger. We did this to allow customization of formatting for flow and task run logs. However, this makes it difficult to change the behavior of Prefect's loggers since you need to adjust multiple handlers. We can accomplish this same feature with a single formatter that has support for flow and task run formats.

Example

Now, since there are not separate formatters for flow and task runs, you can change the format with a single setting e.g. to use JSON logging

PREFECT_LOGGING_HANDLERS_CONSOLE_FORMATTER="json" python example.py

Checklist

• This pull request references any related issue by including "closes <link to issue>"
  • If no issue exists and your change is not a small fix, please create an issue first.
• This pull request includes tests or only affects documentation.
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[rpeden]

This looks great! I tried it and found:

• Everything worked as expected on all Python versions we support.
• A customized logging config based on an old version of logging.yml continued to work the way it worked before.

[peytonrunyan]

It looks good to me, but I'm not as familiar with logging configuration. Got some reading pulled up for later.