feat: Add custom interpretation API

[mrzimu]

This is an idea/design of making uproot able to read custom classes, also allowing users to customize the reading process in uproot.

A summary of the design is:

• Add a registration mechanism for user-defined custom interpretation.
• Add a pre-defined interpretation AsCustom, which
  • Introduces a new layer of interface Reader for converting custom classes to awkward arrays.
  • Defines a series of readers following standard ROOT streaming rules.
  • Allows users to register their own custom readers.

The full description is in dev/custom-interpretation/README.md.

Since I have limited understanding and experience on either ROOT streaming or interface design, it would be grateful if any suggestion or help is provided😄

[ianna]

Here is the link to the discussion: #1424