Add codecov to repo

[MillironX]

Background

This is phase one of my four-part plan to fix the gripes with this action

• Add code coverage metrics (so we know where we came from)
• Separate API calls from install logic (cleanly separating these should greatly simplify the required fixes for Clean up * imports #7, Hitting API Rate limit fails ungracefully #19, Check that Java is installed #24)
• Aside: apply the fixes
• Address Change repo to nf-core/actions #22 (now that we have a clean code base, we can build upon it)

This PR

Add code coverage metrics (so we know where we came from)

Unit testing API calls is hard, especially when the point of this action is to resolve version numbers from an API that are constantly changing. I have working code that can make the entire action unit-testable, but I want to establish a baseline before I submit that.

[edmundmiller]

Love it! Let's call it v2.0.0 when this moves to nf-core actions. I've been wanting to do something like that after seeing https://github.com/emacscollective/workflows