build: add codecov upload

[manmartgarc]

PR Guideline

Typically, PRs should consist of a single commit, and so should generally follow
the rules for Go commit messages.

You must follow the form:

net/http: handle foo when bar

[longer description here in the body]

Fixes #12345

Notably, for the subject (the first line of description):

• the name of the package affected by the change goes before the colon
• the part after the colon uses the verb tense + phrase that completes the blank in, “this change modifies this package to ___________”
• the verb after the colon is lowercase
• there is no trailing period
• it should be kept as short as possible

Additionally:

• Markdown is allowed.
• For a pervasive change, use "all" in the title instead of a package name.
• The PR description should provide context (why this change?) and describe the changes
  at a high level. Changes that are obvious from the diffs don't need to be mentioned.