AWS CodePipeline custom stage to read config file and call external service endpoint

You can use another CodeBuild stage with a standard Linux image provided by AWS (ex: aws/codebuild/standard:6.0 which is an Ubuntu 22.04) and do any commands you need. For example:

version: 0.2
env:
  shell: bash

phases:
  pre_build:
    commands:
      - aws s3 cp s3://<myconfigbucket>/<myconfigfile> # get a config file
      - $(<command to parse config file or any other action needed>)

  build:
    commands:
      - curl -X POST -d 'key1=value1' -d 'key2=value2' https://example.com/fake.php

This is just an example, you can put any bash command you need to match your use case.