ID5 User ID Module - refactor fetch and add server-side config options

[smenzer]

Type of change

• [x] Feature
• [x] Refactoring (no functional changes, no api changes)

This PR mostly refactors the process to request an ID5 ID from our servers, utilizing promises and better organizing the code. In doing so, we've also added a feature to allow config to be pulled down from our server (so publishers don't have to make changes to their website when they want to add features) and an optional ability to add extensions that get passed into the request for an ID5 ID that are used when determining the ID5 ID to provide back to the publisher.

• A link to a PR on the docs repo at https://github.com/prebid/prebid.github.io/
  • https://github.com/prebid/prebid.github.io/pull/3944

[smenzer]

@jlukas79 can you please take a look at this?

[smenzer]

given the approval by @jlukas79 i am merging this