feat: support authorization header for http endpoints

[irealva]

Context
As users have brought up in #19, Flat did not support http endpoints that require authentication.

The original PR was submitted by @4lch4 in #22. I made tweaks to supply the authorization header only when it is not undefined and removed the version bump which is now outdated.

Fix
This PR adds an authorization param to the Flat action that requires a string and can be used to supply authentication information in the following way:

with:
    http_url: https://api.twitter.com/1.1/trends/place.json?id=1
    authorization: "Bearer ${{secrets.TWITTER_API_BEARER_TOKEN}}"
    downloaded_filename: twitter/trending-global-hashtags.json
    postprocess: ./src/processors/Twitter.ts

[idan]

Minor change needed in action.yml but otherwise LGTM!