Add example values to OpenAPI generator (#708)

[gjavakhadze]

Description

Add support for samm:exampleValue in OpenAPI and AsyncAPI property schemas.

Fixes #708

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works

[e-filchenko-bosh]

LGTM!

[atextor]

LGTM!