Access metadata: load properties and expose them

[Tristramg]

This pull request should be readable commit after commit (each language separately)

[Castavo]

Tested with python, works very fine.

Don't know how to test wasm bindings, could you help me ?

The code looks clean, it's weird that the Properties type belongs to the builder module and not the lrs module, but that was there before, so it's okay I guess

[Tristramg]

I moved out Properties to put it in the Lrs module

To test wasm, you could add some console.log in the demonstrator to be sure that we read the data. Maybe I should learn how to make a node script to avoid going through all the navigator stuff, but I’m not confident for that.