OpenAPI 3.1.0 support: add support for Lincense.identifier field

[char0n]

The goal of this issue is to add support for License.idenfitier field.

Proposed solution: nothing changes from rendering perspective.

If definition author supplies License.url field we render it as before.
If definition author supplies License.identifier field, we compute SPDX url and render it as before
If definition author supplies both License.url and License.identifier, License.url is used to render the link

[char0n]

If License.identifier is provided, renders as following image with this link: https://spdx.org/licenses/Apache-2.0.html