Add Gitee Backend in Grimoirelab

Hello, everyone! We want to merge the grimoirelab-gitee（[perceval](https://github.com/grimoirelab-gitee/grimoirelab-perceval-gitee), [enrich](https://github.com/grimoirelab-gitee/grimoirelab-elk-gitee)） solution back to Grimirelab in CHAOSS, just as discussed in slack grimoirelab channel.
In our previous solution, we made grimoirelab-gitee as a [plugin](https://github.com/chaoss/grimoirelab-elk/issues/835#). Now we plan to push it into main branch as one of backends. Instead of plugin solution, we will follow the current backend structure, avoiding legacy broken.
We will first put perceval-gitee into the main branch and then enrich part into elk. @eyehwan and I will provide long-term maintenance work for it.
In addition, the plugin solution can be used to incubate a new data source backend solution, just like what we have done for grimoirelab-gitee as an independent project in a separate github repo. When it's mature and stable enough(one-year incubation for example), we can consider merging it back to Grimoirelab. We can provide a detailed guideline for it.

So I would like to launch a discussion here and we could talk more about the details of this plan.@vchrombie,@sduenas, @dicortazar, @sgoggins, @GeorgLink
Thanks!