Implement SDF 1.8 specification

## How to participate
💬⬇️ Comment below and mention the struct or Enum you'd like to implement.

This will ensure that there is no overlap in the PRs. Everybody will be happy 😊

📃 There should be written tests to make sure that the specification aligns with the implementation.

⚠️ Because of the way XML works, not distinguishing between an attribute `<... this-is-attribute="Value" >` and an element `Value`, you must be careful when writing the tests to make sure that the deserialized values match with the result you're expecting.

✔️ Open a PR when you're ready or if you're stuck or need feedback on your implementation.

## Specifications
The XML specifications can be found on the official website:
http://sdformat.org/spec?ver=1.8

## TODOs (Non-exhausive list):
of structs & enums that need to be implemented:

- [ ] Actor
- [ ] Light @hannydevelop
- [ ] Scene
- [ ] State @hannydevelop
- [ ] Physics @hannydevelop
- [ ] Link
- [ ] Sensor
- [ ] Joint
- [ ] Todo: add more structs