Add JSON Schema for monitoring data-type [#72764046]

- Adds monitoring data-type to the fixtures
- Adds new schema 'stagecraft/apps/datasets/schemas/monitoring.json' with `uptime` and `downtime` as the only required fields (integer)
- Extends 'stagecraft/libs/schemas/schemas.py' to build a series of $ref references 
  - these are passed to the 'allOf' combinator
  - uses a `DEFINED_SCHEMAS` list, this feels a bit dorky, but we probably aren't going to define *that* many

General feeling trying to get the tests working throughout (probably suggesting that I could've gone about this in a very different way) is well described by the following:

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