Should discriminator emit a warning oif property does not exist

[timotheeguerin]

Could kinda solve this issue too #2583

We didn't use to require the discriminator property on the base type originally because we didn't have property override but now we do it feels like you should describe that property.

//bad
@discriminator("kind") model Pet {}

// good
@discriminator("kind") model Pet { kind: string }

[markcowl]

• core: require property?
• azure: suggest enum value for property?

[timotheeguerin]

Decision create azure-core linting rule that validate discriminator is a union so it would take care of that. And instead investigate issue #2737