rf(metaschema): Validate object definitions that are JSON schema

[effigies]

schema.objects.metadata entries are very nearly JSON schema definitions. To allow BIDS validators to stand on the shoulders of JSON schema validators, the goal of this PR is to define them as JSON schema with extra fields. By doing so, BIDS validators can pass these directly to JSON schema validators along with the pieces of JSON objects that they are intended to validate.

In order to do this, I've tried to formalize the different "kinds" of objects (or terms) we have in composable classes.

The main change to the schema is to rename required_fields to required, which has the same meaning in JSON schema. Similarly, we update recommended_fields to recommended, although this will have no impact on validation for now.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 88.04%. Comparing base (6beec13) to head (e78af92).
Report is 292 commits behind head on master.

Files with missing lines	Patch %	Lines
tools/schemacode/bidsschematools/render/tables.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1869   +/-   ##
=======================================
  Coverage   88.04%   88.04%           
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  Files          16       16           
  Lines        1380     1380           
=======================================
  Hits         1215     1215           
  Misses        165      165           

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[effigies]

Validating objects.metadata as JSON-schema+ objects finds no failures. @rwblair This should be relatively straightforward to review, but LMK if you want to go over anything live.

I've got some more stuff that does catch bugs in the schema that I want to patch in after this is in.