[Task] Document design decision for JSON payloads

[atextor]

Is your task related to a problem? Please describe.
The section Payloads documents how JSON payloads are mapped to Aspect Models. Currently no decision record documents why the payload is defined like this, and why it intentionally does not contain any references to the models.

Describe the solution you'd like
A decision record should be created that details:

• SAMM's approach of cleanly separating between meta model layers (M0/M1/M2/M3) as described in OMG's MOF (i.e. ISO/IEC 19502:2005 / ISO/IEC 19508:2014)
• Pros and Cons of the approach in comparison to self-describing data or other systematic references from runtime data to the model (e.g. JSON-LD), i.e. backwards compatibility to existing APIs, straightforward APIs (OpenAPI generation), mapping of multiple model versions to data endpoints etc.

Additional context
The topic was already adressed/discussed

• in this forum post
• in [Task] Add descriptive auto generated JSON payloads esmf-sdk#491