[Potential addition to the CLI] Ability to re-use an existing report, if we have one in storage from another commit

[vlad-ko]

What product do you want to improve? General processing of the reports by Codecov

Is your feature request related to a problem? Please describe. One of our customers has the following workflow:

1. PR is opened
2. All affected subprojects are built and unit tests run (this report has all the required coverage info to establish a solid base)
3. Coverage report uploaded
4. PR merged
5. All affected subprojects are built and unit tests run
6. Coverage report uploaded

They are trying to cut down on the redundancy of 5/6 since the coverage data is the exact same before the merge happens, and we haven't had issues with stale coverage data so far

Describe the solution you'd like Add a command to codecov-cli that would do something like "re-use" report from <SHA>. This would save the customer the need to run tests and do an upload, and will simply re-use the existing report from storage. I know we have an empty-upload command, but it seems to serve a slightly different purpose.

Describe alternatives you've considered Leverage empty-upload command, but make sure we re-run all of the required calculations and can essentially establish a new base from this upload.

Additional context To summarize what they really want is:

1. Some way to fetch the coverage report in step 3,
2. Some way to do step 6 without having to use the Codecov upload binary or the codecov-cli