Allow custom prefix

[RudyWeber]

Some tools such as create-react-app only expose specifically prefixed environment variables (for CRA, it's REACT_APP_) and do so only at build time. Since Create React App produces a static HTML/CSS/JS bundle, it can’t possibly read them at runtime.

This is problematic when we need access to PLATFORM_* environment variables.
To do so, we need to be able to specify a custom prefix so we can pick up the re-exported relevant variables with the prefix.

Then we can do:

.platform.app.yaml

hooks:
  deploy: |
    set -e
    REACT_APP_PLATFORM_ROUTES=$PLATFORM_ROUTES \
    REACT_APP_PLATFORM_RELATIONSHIPS=$PLATFORM_RELATIONSHIPS \
    yarn build

somewhere.js

import { config as platformConfig } from "platformsh-config";

// use React-specific prefix
const config = platformConfig({ varPrefix: "REACT_APP_PLATFORM_" });
const {
  environmentVariables: { NODE_ENV },
  isDev = NODE_ENV === "development",
} = config;

const getApiUrl = () => {
  // using platform?
  if (config.hasRelationship("api")) {
    const { host, port } = config.credentials("api");

    // local dev with ssh tunnel
    if (isDev) {
      return `http://${host}:${port}`;
    }

    // prod, we need the api route url
    const url = Object.entries(config.routes()).find(
      ([, { upstream }]) => upstream === "api"
    )[0];

    return `${url}:${port}`;
  }

  // local dev with local server
  return `http://localhost:4000`;
};

[chadwcarlson]

Please also add an entry for this to the changelog