Support configuration variables in Hardhat config everywhere it makes sense

[kanej]

The new version of configuration variables should be usable at any point in the result object of hardhat.config.{js,ts}. Currently the types constrain this.

We should also consider methods of supporting patterns that are the equivalent of:

{
  url: `https://alchemy.com/api?key=${env.API_KEY}`
}

A suggestion was to support templating maybe with a template library:

{
  url: config.variable("https://alchemy.com/api?key={{API_KEY}}")
}

Though I would consider template literals:

{
  url: config.variable`https://alchemy.com/api?key=${"API_KEY"}`
}

function configurationVariableResolver(name: string): string | null {
  if (name === "API_KEY") {
    return "secret"
  }

  return null
}

const config = {
  variable: (strings: TemplateStringsArray, ...keys: string[]): string => {
    return strings.reduce((result, str, i) => {
        const key = keys[i - 1];
        
        return result + configurationVariableResolver(key) + str;
    });
  },
}

const example = {
  url: config.variable`https://alchemy.com/api?key=${"API_KEY"}`
}


console.log(example)
// {
//  "url": "https://alchemy.com/api?key=secret"
// } 

The template literal example here would need to be changed to return a ConfigVariable object so it could be resolved later, but the principle stands.