Centralize GitHub action for running playwright tests

[mickmister]

We should probably have a reusable action to run playwright tests, probably defined here https://github.com/mattermost/actions/tree/main/plugin-ci

[mickmister]

For allowing plugin projects that use these workflows to inject environment variables, I think we can have a PLUGIN_CI_ENV workflow input, that then gets injected into the workflow via an environment variable of the same name. That way plugin projects can provide arbitrary environment variables to its own scripts when running the playwright test. No sensitive env vars will be used here.