hostRules: support dynamic username/password/token fields in config.js

[rarkins]

Describe the proposed change(s).

Some users have the need to use rotating credentials. We should enhance hostRules so that username, password and token fields can be a function, which Renovate awaits on before returning.

Example config.js after:

let githubToken;

async function getGitHubToken() {
  if (githubToken) {
    return githubToken;
  }
  githubToken = // some fetching logic;
  // delete cached token after 30 minutes
  setTimeout(() => {
    githubToken = undefined;
  }, 30*60*1000);

}

module.exports = {
  hostRules: [{
	matchHost: 'github.com',
  }]
}

hostRules functions in lib/util/host-rules which can return credentials (such as find, findAll) should be modified to resolve such fields before returning them when they're a function like this. Such function will themselves now need to be awaitable.

Internally we should keep the HostRule type as having strings, but add a new InternalHostRule type we use for storage which allows strings or functions.

[rarkins]

I experimented with implementing this but found that changing all hostRules find/get functions to async added a lot of complexity.