Add support for send-notifications in codecov-action

[ndmanvar]

Currently you need to use cli/binary to send-notifications in GitHub Action.
Codecov GitHub Action isn't setup to call/pass send-notifications to underlying cli.

Ask here would be to have functionality (and supporting docs) for an action to manually notify codecov of upload completion.

[ryanhiebert]

I would appreciate this. I'm trying to use OIDC, and I want to manually tell codecov when all my coverage-uploading jobs have completed, so doing all this through the action would be optimal. I'm not sure if I can use OIDC through a manually-installed CLI, and I'd rather not dig in to find out, but I'd absolutely use it if I could do it here in the action.

[ascopes]

This would be appreciated by me too, my pipelines run a number of parallel builds and aggregate coverage using codecov. Each build targets a slightly different platform where coverage varies due to platform specific code being under test.

I'd like to defer notifications for codecov until these builds have all finished to prevent my email inbox being spammed by comments that immediately get updated again as each build finishes.