Support codecov

[La0]

As it needs a token for upload. Nice to have.

[calixteman]

For rust, an example of what we're using:
https://github.com/mozilla/dump_syms/blob/master/.taskcluster.yml#L80-L82
And require we build with rust nightly and these options:
https://github.com/mozilla/dump_syms/blob/master/.taskcluster.yml#L66-L67

[La0]

The issue here is that you need grcov to convert the payload.
I don't think task-boot should depend on grcov: it should expect an archive to upload to codecov, only managing the upload with secrets.

As for a frontend build + upload, we should have 2 tasks:

1. implemented fully by the client app to produce a codecov.zip artifact
2. taskboot codecov

@marco-c Any thoughts as that touches grcov ?