Using per-type base classes for Metadata

In order to properly expose input type in __init__ (for pyright and user assit),
use one base class (subclassing Metadata) per input type.

Cant get rid of the `Any` on `Metadata` init (otherwise would me re-implement the init everywhere).

Used the opportunity to remove the `expecting` classvar and modified tests accordingly

- Also fixed a minor issue in bytes reading by seeking back to previous position and not zero.
- Also shared binary reading logic inside main base class (was already there) so it can be reused in illustration
- Now explicitly says the type of stored data (can be different to inputs in somewhat flexible ones)

Author: rgaudin