[2.0] Separate AIND-specific functionality into aind_extensions.py

[dbirman]

Describe the solution you'd like
AIND functionality goes into the aind_extensions. A custom validator can be written that accepts a Metadata object and checks that the appropriate AIND extensions are in place

• acquisition
• data_description
• instrument
• metadata
• procedures
• quality_control
• rig
• session
• subject

Additional context
https://alleninstitute.sharepoint.com/:w:/r/sites/NeuralDynamics/_layouts/15/Doc.aspx?sourcedoc=%7BE27B889D-27A1-4CC0-948C-10ADBE605654%7D&file=Data+Schema+2.0.docx&action=default&mobileredirect=true&DefaultItemOpen=1