feat: implement Asset specialization

[paullatzelsperger]

What this PR changes/adds

• added a method Asset#isCatalog()
• Management API ingress: JSON-LD transformers parse the @type field and set the Asset#properties['isCatalog'] flag
• Management API egress: JSON-LD transformers set the @type field on egress if Asset#properties['isCatalog'] is true
• the information whether an Asset is a CatalogAsset is stored in Asset#properties

Why it does that

prepare for the next iteration, where Dataset extends Catalog

Further notes

• I had originally planned to store the information in the privateProperties, but that would have made querying for CatalogAssets impossible, because we currently can't query by private properties. If we don't care about querying, I'd suggest moving the flag back to the privateProperties
• the EqualOperatorPredicate can now handle booleans

Linked Issue(s)

Closes #4295

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