[REQUEST] Ensure external_references' urls are RFC3986 Compliant

Is your feature request related to a problem? Please describe.

STIX 2.1 requires that the Common Data Type of External Reference (external_references) have their url field be RFC3986 compliant (link to STIX 2.1 reference). Ensuring this in the ATT&CK Data Model would help ensure that URLs stay in compliance with STIX 2.1 requirements.

Describe the solution you'd like

This could be added to the zod schema here: https://github.com/mitre-attack/attack-data-model/blob/main/src/schemas/common/property-schemas/stix-external-references.ts

Describe alternatives you've considered

Alternatively we could not bother with this enhancement since it is STIX 2.1 specific, but if not here, then where!?

Additional context

This has bitten the ATT&CK dataset in the past, e.g. here: https://github.com/mitre-attack/attack-stix-data/issues/16

The referenced issue has some good examples of what was historically included in the STIX bundles before they were manually corrected

Start with src/schemas/common/property-schemas/stix-external-references.ts and review the RFC3986 and STIX 2.1 references in the issue. Check the historical examples in attack-stix-data issue 16, then verify that external_references URL values are accepted or rejected according to the requested compliance requirements.