Add Role enum to openai module

[declark1]

Description

We should add a Role enum to the clients::openai module to be used instead of String. It should also include the new developer variant.

Acceptance Criteria

• Role enum is added to openai module with variants for user, assistant, developer, system, tool and #[serde(rename_all = "lowercase")] attribute
• All role: String fields have been updated to role: Role
• All usage has been updated accordingly