[ENH] Additional age related columns for participants.tsv file to cover animal data

[ree-gupta]

The PR adds columns for age_category, age_type, birthdate as discussed and suggested by @yarikoptic in #1826

This is open for discussion.

Related issues:

• #1705
• #1634
• #1791

@bids-standard/bep032

TODO:

• [x] Add the info that either birthdate or age should be specified - decision to keep birthdate as optional and if age is specified, AgeReference must be used in sidecar JSON
• [x] Feedback on term name to include 'stage' to match UBERON - deviate slighly from UBERON and not use 'stage' in the enum names
• [x] Confirm introduction of AgeReference rule in schema for participant.tsv
• [x] Which key to use for age column definition that would specify that one must use AgeReference in sidecar?
• [ ] Default for 'AgeReference'
• [ ] Add rule that either birthdate or age must be specified
• [ ] Formalized 'TermURL' as suggested by @yarikoptic here - ??
• [x] Resolve conflicts (definition was used - different from description and enums use)