Better link validations in schemas

[e-backmark-ericsson]

Description

In PR #380 some validation of link types was added to the json schemas. There are even more link type validations that could be implemented though, such as:

• Multiple links of the same type is allowed or not - would benefit from uniqueKeys as described here: Key-based item uniqueness json-schema-org/json-schema-vocabularies#22
• Link types not available should not be possible to use, or? Should we instead provide an optional customLinks object, similar to customData but for links?

Motivation

Better event validations

Exemplification

.

Benefits

See motivation

Possible Drawbacks

Disallowing custom link types from the links object would be a non-backward compatible change