fix(view-engine): define metadata for SlotCustomAttribute

[StrahilKazlachev]

fix #566
@EisenbergEffect I have some concerns about the fix, since ViewEngine creates and initializes the behavior instance:

• currently the relation is au-slot behavior instance per ViewEngine instance.
• defining the metadata changes this - now it's on the class SlotCustomAttribute.
  • is it supported use case to have more than one instance of ViewEngine in an app(enhance)?
  • is it supported use case to have more than one apps running in parallel?

[EisenbergEffect]

I think this will work. I'm going to move the first three lines out of the instance so the same behavior would be shared. Init is guarded so that it's only run once.