aws: refactor AWS plugin (#7615)

* Update the AWS plugin to support disabling RPROMT display:
  Use a $SHOW_AWS_PROMPT option.

* Refactoring aws plugin:
  Exposing customizable aws_prompt_info function to be used in themes.

* Set aws prompt prefix and suffix to original values and fix README

Co-authored-by: "Vassilis S. Moustakas" <vsmoustakas@gmail.com>