feat(openapi): add support for server variables

[cristicbz]

Add support for server variables like:

            .server(
                ServerObject::new("https://{environment}.example.com/{organization}/{tenant}/api/v1")
                .enum_variable(
                    "environment",
                    "The environment to call (e.g. alpha).",
                    "alpha",
                    vec!["alpha", "staging", "cloud"],
                )
                .variable(
                    "organization",
                    "Organization name or UUID",
                    "00000000-0000-0000-0000-000000000000",
                )
                .variable(
                    "tenant",
                    "Tenant name or UUID",
                    "00000000-0000-0000-0000-000000000000",
                )
            )

I didn't find a very natural place to add tests for this, please let me know if there is a good place to do so.