add openMINDS_schemaValidation action / workflow

[lzehl]

openMINDS_core even has an old openMINDS_validation action / workflow which should be replaced/enhanced with a new one for schema validation (checking on JSON syntax, maybe additional checks such as required properties appear actually in the properties, etc).

This action / workflow can either be added to the submodules or to main.
@openMetadataInitiative/openminds-developers what are your thoughts?

[lzehl]

Common errors we should catch:

• JSON syntax failures
• capitalization errors for attypes
• mismatch between required property terms and actual property terms
• syntax error for _extents paths
• ???

[lzehl]

add all possible checks to submodules.
join code for all actions into one repo (openMINDS_actions)

main only: extension builds, category builds