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.