feat(discovery): add kind field in order to discover the check type

[FlorentinDUBOIS]

Resolve #4.

[urcadox]

Will review that tomorrow :)

[urcadox]

Alright so, 2 things:

• Only adding a kind field to the existing (http) check does not really work. The existing Check case class has properties specific to the http check (secure and path).
• If we were to do this, the kind field should be an Enum, something like this:

sealed trait CheckKind

case object HTTP extends CheckKind
case object DNS extends CheckKind
case object SSL extends CheckKind

Here though, what we have to do is have a Check trait and multiple case classes which implement this trait.

Then, when serializing, use pattern matching to add the relevant kind property.

Database-wise though, it would be best to use separate tables for every check kind, though we could get away with using a single table with a few constraints to keep it clean.