Identity enum in v2 common types - only has SystemAssigned? What to do about UserAssigned?

Today the ResourceIdentityType 'string enum' in CommonTypes doesn't have 'UserAssigned' as a valid enum value. Or the combination 'UserAssigned, SystemAssigned'. The tricky bit as I see it is that many services would probably like it to define the other values with 'UserAssigned' also, and then they can't really do that by referencing 'Identity' in commonTypes - or can they?

(And if they *were* added in common-types that would look like a breaking doc/contract change for any RP using it today right?)

```
"Identity": {
"description": "Identity for the resource.",
"properties": {
"principalId": {
"readOnly": true,
"type": "string",
"description": "The principal ID of resource identity."
},
"tenantId": {
"readOnly": true,
"type": "string",
"description": "The tenant ID of resource."
},
"type": {
"type": "string",
"description": "The identity type.",
"enum": [
"SystemAssigned"
],
"x-ms-enum": {
"name": "ResourceIdentityType",
"modelAsString": false
}
}
}
```

Start by reviewing the Identity definition in CommonTypes and searching for ResourceIdentityType usages across the specifications. Determine whether UserAssigned and combined identity values can be represented without breaking existing resource-provider contracts. Done means the compatibility concern and the supported contract approach are resolved, with the relevant specification changes identified.

Written by the indexing model from the issue text.