Add support for background to CloudEvent conversion

[mtraver]

No description provided.

[mtraver]

Unit tests pass but coverage is at 99.17%.

Lint says:

lint run-test: commands[1] | isort -rc -c src tests setup.py conftest.py
ERROR: /home/runner/work/functions-framework-python/functions-framework-python/src/functions_framework/convert.py Imports are incorrectly sorted and/or formatted.
ERROR: /home/runner/work/functions-framework-python/functions-framework-python/src/functions_framework/__init__.py Imports are incorrectly sorted and/or formatted.
ERROR: /home/runner/work/functions-framework-python/functions-framework-python/tests/test_convert.py Imports are incorrectly sorted and/or formatted.
/home/runner/work/functions-framework-python/functions-framework-python/.tox/lint/lib/python3.9/site-packages/isort/main.py:1133: UserWarning: W0501: The following deprecated CLI flags were used and ignored: -rc!
  warn(
/home/runner/work/functions-framework-python/functions-framework-python/.tox/lint/lib/python3.9/site-packages/isort/main.py:1137: UserWarning: W0500: Please see the 5.0.0 Upgrade guide: https://pycqa.github.io/isort/docs/upgrade_guides/5.0.0/
  warn(
ERROR: InvocationError for command /home/runner/work/functions-framework-python/functions-framework-python/.tox/lint/bin/isort -rc -c src tests setup.py conftest.py (exited with code 1)

[mtraver]

Ok, all checks pass! Yay!

[grant]

Nice 🥳

Please look at the review comments – there are some requested minor changes.
No major blocking comments, so approving. LMK if any comments could be clarified.