Support trend graph rendering for test coverage

When running a test coverage task using "gcovr" in an Evergreen, XML or HTML output could be generated, which provides users a snapshot of code coverage information for that specific build/version. A trend graph of those code coverage data would be useful to observe change dynamics over time, to help identify testing gaps for improvement. 

The Evergreen perf plugin supports the rendering of trend graphs, but it requires a specific format. Mongo server has a website to track code coverage for weekly/daily builds, and it does not provide trending capability either. 

Jenkins CI has a Code Coverage API plugin that takes XML inputs and renders trend graphs, which could be used as a reference.