[backends] How to keep a track of APIs

This issue is opened as GrimoireLab does not have a way to keep track of the changes which happen to the APIs used to fetch the data from various data sources. Although this does not present itself as a big issue as most of the APIs are quite stable and changes do not take place much often. However, some APIs are still in there `beta` phase(e.g. [Gitter](https://developer.gitter.im/docs/welcome)) and may evolve in the future.

A bit of discussion about this was made here: https://github.com/chaoss/grimoirelab-perceval/pull/636

EDIT: Another reason for opening this issue is that a lot of the APIs may add additional fields which may help us in getting more data from there data sources. However, this would not come in light unless the documentation is checked.

Start by reading the issue and the discussion in pull request #636, then review how Perceval's backends fetch data from external APIs. The desired tracking approach and scope are not defined; completion would require an agreed design for detecting API changes and handling newly available fields across the relevant data sources.

Written by the indexing model from the issue text.