Active Skills

**Is your feature request related to a problem? Please describe.**
Get servant active skills data.

**Describe the solution you'd like**
A servant data looks something like this
```
{
name: "Gilgamesh",
class: "archer",
...
}
```

I would like to have references along the lines of this
```
{
...servant
skill: {
active: [
{
skillId: id,
upgradesTo: someOtherId
}
]
}
}
```
and a skill would look something like this
```
{
id: ""
name: "",
description: "",
growth: [ { level: 1, cooldown: 7, effect: ""} ... ]
}
```

**Describe alternatives you've considered**
If you think you have a better one, feel free to describe it in a comment. What I propose is just a draft of something I had considered.