OpenAPI spec: Naming conventions

[agraebe]

As part of the OpenAPI spec review, we realized that there is a lack of naming conventions. It would be useful to extend our contributions guidelines with some initial conventions based on the Google's Cloud API conventions. Some points we should cover:

• case styles
• abbreviations
• singular vs plural
• underscore vs hyphen

@rafaelcr @zone117x @pgray-hiro - is there anything that I missed?

@kyranjamie would you be able to contribute with some thoughts on this?