Modernizes, adds tests, fixes warning and add API key auth (Closes #4 and #5)

[zedtux]

This PR reworks many parts of this gem in order to:

• Modernize it (Adds standard missing files, like Rakefile, converts the README file to Markdow, adds Docker/Earthly, etc ...)
• Adds nokogiri as a dependency to the gemspec file
• Fixes passing a different sender to the sms method
• Adds test using RSpec to ensure not break the current behavior and adds Github Actions to get the test suite running on each commits/PR (You can have a look from our repository
• Fixes the warning reported from issue RestClient is showing "warning: Overriding "Content-Type" header "application/json" with "application/x-www-form-urlencoded" due to payload" #4
• Implements the API key authentication as requester per Add support for API key authentication #5

[zedtux]

I just spotted an issue (data error) I'm going to quickly fixe.

[zedtux]

The issue is gone, I received the SMS.