Make the Coverage API more understandable and easier to use

**What it's the actual situation on ARA ?**
I wanted to use the api /functionalities/coverage to retrieve information about the coverage in my team.
The response is not user-friendly and it's hard to know how datas are used to build the hmi

**Describe the solution you'd like**
A response more friendly. something like (it's just an example)
{"teams": "15,
"severity" : "High",
"coverage" : {
"Covered" : 3,
"Ignored" : 4,
......},
{"teams": "15,
"severity" : "Medium",
"coverage" : {
"Covered" : 3,
"Ignored" : 4,
......},
....

**Describe alternatives you've considered**
NA

**Additional context**
NA