Clarified $id attribute

[mitfik]

According to to JSON schema specification $id should be an URI this would make some tooling fails if we would specify SAID as an $id.

Not all tools enforced that so the tools overall are not consistent neither.

Potential solution:

• clarify that the ACDC is not consistent with JSON Schema specification in that regards
• define in ACDC that $id needs to be SAID schema to stay consistent with URI specification e.g. said:Eo8uTTa8TAhXNn_hTM6b9yHK49VcH5Wh5JT5uuFu3CWk
• propose to JSON schema to allow for none URI values of the $id attribute

[mitfik]

Seems that we don't have to do anything on that one: https://datatracker.ietf.org/doc/html/rfc3986#section-4.2

SAID should be easily treated as Relative Reference and all parser should fine with it.