[EventGrid] API design issues for the `identity` block

👋

The Swagger definitions for Identities within the EventGrid API ([this Swagger](https://github.com/Azure/azure-rest-api-specs/blob/921957dfa2228c2062b8d2ef832507339c5562d4/specification/eventgrid/resource-manager/Microsoft.EventGrid/preview/2020-04-01-preview/EventGrid.json#L1)) appear to disregard the conventions used for `identity` blocks elsewhere in this codebase, such that a normal Identity block gets generated as:

```
// ManagedServiceIdentity managed service identity.
type ManagedServiceIdentity struct {
// Type - Type of managed service identity. Possible values include: 'ManagedServiceIdentityTypeSystemAssigned', 'ManagedServiceIdentityTypeUserAssigned', 'ManagedServiceIdentityTypeSystemAssignedUserAssigned', 'ManagedServiceIdentityTypeNone'
Type ManagedServiceIdentityType `json:"type,omitempty"`
// TenantID - READ-ONLY; Tenant of managed service identity.
TenantID *string `json:"tenantId,omitempty"`
// PrincipalID - READ-ONLY; Principal Id of managed service identity.
PrincipalID *string `json:"principalId,omitempty"`
// UserAssignedIdentities - The list of user assigned identities associated with the resource. The user identity dictionary key references will be ARM resource ids in the form: '/subscriptions/{subscriptionId}/resourceGroups/{resourceGroupName}/providers/Microsoft.ManagedIdentity/userAssignedIdentities/{identityName}
UserAssignedIdentities map[string]*ManagedServiceIdentityUserAssignedIdentitiesValue `json:"userAssignedIdentities"`
}
```

but EventGrid instead generates:

```
// EventSubscriptionIdentity the identity information with the event subscription.
type EventSubscriptionIdentity struct {
// Type - The type of managed identity used. The type 'SystemAssigned, UserAssigned' includes both an implicitly created identity and a set of user-assigned identities. The type 'None' will remove any identity. Possible values include: 'SystemAssigned', 'UserAssigned'
Type EventSubscriptionIdentityType `json:"type,omitempty"`
// UserAssignedIdentity - The user identity associated with the resource.
UserAssignedIdentity *string `json:"userAssignedIdentity,omitempty"`
}
```

it's also worth noting the value defined above "None" isn't present in as a Constant, even though it's mentioned above:

```
// EventSubscriptionIdentityType enumerates the values for event subscription identity type.
type EventSubscriptionIdentityType string

const (
// SystemAssigned ...
SystemAssigned EventSubscriptionIdentityType = "SystemAssigned"
// UserAssigned ...
UserAssigned EventSubscriptionIdentityType = "UserAssigned"
)
```

Would it be possible for this to be fixed to match other API's? This appears to be both a design issue (in the API) and a process issue (that this slipped through) - do common types not exist for the `identity` block?

cc @JeffreyRichter

Start with specification/eventgrid/resource-manager/Microsoft.EventGrid/preview/2020-04-01-preview/EventGrid.json at the linked identity definitions, then compare them with identity blocks elsewhere in the repository. Confirm the EventGrid schema follows the established conventions and that every declared identity type, including None, is represented consistently in the generated output.

Written by the indexing model from the issue text.