Add ec3.3 draft schema

[hl662]

This pull request introduces a new JSON schema definition for the EC3.3 draft representation of an ECSchema. The schema establishes the structure and validation rules for ECSchema objects, including required properties, references to external definitions, and constraints on additional properties.

The only changes between the 2 schema.json files in this ec3.3 draft and ec3.2 are the $id and $ref values that previously pointed to ec/32, updating them to ec/33.

[diegoalexdiaz]

I see two ways of evolving this draft on our repo:

1. Introduce the draft as proposed into master - as a sibling folder of the released 3.1 and 3.2 specs, with the -draft suffix. Drop that suffix once we release it.
2. Keep evolving this draft on a branch and only merge it once it is ready to be released.

@ColinKerr - are there other approaches to handle this?