feat: add whoami composite action

[nicomiguelino]

Closes #8

Summary

Extracts the inline whoami shell step into a reusable composite action at Screenly/edge-apps-actions/whoami@v1.

Consuming repos can now replace the inline shell block with:

- uses: Screenly/edge-apps-actions/whoami@v1
  with:
    screenly_api_token: ${{ secrets.SCREENLY_API_TOKEN }}
    api_base_url: ${{ vars.API_BASE_URL }}

Changes

• Add whoami/action.yml composite action that logs the current user and team via the Screenly API
• Update README.md with usage example and input reference

[Copilot]

Pull request overview

This PR introduces a reusable whoami composite GitHub Action that calls the Screenly API to print the current user and team, and documents how to consume it from other repositories (closing #8).

Changes:

• Added a new composite action at whoami/action.yml to fetch and log current user/team from the Screenly API.
• Updated README.md with usage and input documentation for the new whoami action.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
whoami/action.yml	Adds the new whoami composite action that calls Screenly API endpoints and prints user/team info.
README.md	Documents the new action, including an example workflow snippet and input table.

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[renatgalimov]

Please, fix it right away.

[renatgalimov]

Then, once @salmanfarisvp or @rusko124 approves it - feel free to land it.