`identifier` field for License Objects

License Objects should have an identifier field, which can hold Software Package Data Exchange license identifiers 😄

From the SPDX Specification:

Identifying the license for open source software is critical for both reporting purposes and license compliance. However, determining the license can sometimes be difficult due to a lack of information or ambiguous information. Even when licensing information is present, a lack of consistent notation can make automating the task of license detection very difficult, thus requiring vast amounts of human effort.

https://spdx.org/spdx-specification-21-web-version#h.twlc0ztnng3b

Rationale

The OpenAPI Specification, as we all know, is meant to be machine and human readable1.

Adding a standardized, machine-readable license field would allow for the following:

• more precision w/r/t specific licenses, without the brittleness of a url field (URL rot is a thing!)
• automatic linking to common licenses in documentation tools, even if the author omits a link
• an index that categorizes APIs by license without relying on license.name string comparison
• an OpenAPI client that automatically checks for compliance with services it interacts with

[MikeRalphson]

#2105 was merged into v3.1.0-dev, look out for an RC-0 release soon.

[MikeRalphson]

@lorenzleutgeb I’ve personally found that creates confusion with the Unlicense license. I think omitting an identifier and relying on the name/url would be preferable. Why would there not be a url for a proprietary license?