Feature/one of support

[z6n9n]

What kind of change does this PR introduce? (check at least one)

• [ X ] Bugfix
• [ X] Feature
• [ X ] Test
• Docs
• Refactor
• Build-related changes
• Other, please describe:

Description: OAS 3 provides support for oneOf with 'discriminator' and the 'example' keywords. Newer AJV versions support this too. This change adds the support for it to the project.

https://swagger.io/docs/specification/data-models/inheritance-and-polymorphism/

Unfortunately AJV does not yet support the 'mapping'-property of 'discriminator' as OAS3 does, so I had to remove the mapping from the AJV schema object. Validation support for discriminator is still possible without this mapping property.
See: ajv-validator/ajv#2003

Example OAS Code:

oneOf: //supportet by AJV
  - $ref: '#/components/schemas/Object1'
  - $ref: '#/components/schemas/Object2'
discriminator: //supportet by AJV
  propertyName: objectType //supportet by AJV
  mapping: //NOT yet supportet by AJV
    obj1: '#/components/schemas/Object1'
    obj2: '#/components/schemas/Object2'

[z6n9n]

after dependency update, test are not anymore supported on node 10 & 12. Node 18 added instead

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication