PYTHON-5467 Add codecov integration

[blink1073]

PYTHON-5467

Changes in this PR

Add codecov integration for pymongo. The GitHub Action portion ties the coverage to a PR, and the evergreen portion adds coverage data for other variants.

Test Plan

Test codecov interaction with the PR.

Checklist

Checklist for Author

• Did you update the changelog (if necessary)?
• Is there test coverage?
• Is any followup work tracked in a JIRA ticket? If so, add link(s).

Checklist for Reviewer

• Does the title of the PR reference a JIRA Ticket?
• Do you fully understand the implementation? (Would you be comfortable explaining how this code works to someone else?)
• Is all relevant documentation (README or docstring) updated?

[codecov-commenter]

⚠️ JUnit XML file not found

The CLI was unable to find any JUnit XML files to upload.
For more help, visit our troubleshooting guide.