`az ad app permission grant` should preserve existing `oauth2PermissionGrant`

## Symptom

In the current `az ad app permission grant`, to order to keep **idempotence**, before creating `oauth2PermissionGrant`, existing grants with matched `clientId` and `resourceId` are removed:

https://github.com/Azure/azure-cli/blob/fd4b5f12d29e6b23c0ebbc7adb207c8c9fd75d2e/src/azure-cli/azure/cli/command_modules/role/custom.py#L1012-L1013

This deletes too many `oauth2PermissionGrant`s as Graph API does support multiple grants with the same `clientId` and `resourceId`:

```
> az ad app permission list-grants --id 2636614c-010c-49b2-ac23-f61d80a5b529 --show-resource-name
[
{
"clientId": "b25985a8-9d2b-4fde-9fa7-6c4e43a8ddec",
"consentType": "AllPrincipals",
"id": "qIVZsiud3k-fp2xOQ6jd7InI76O38TJFngGR4y0QOfQ",
"principalId": null,
"resourceDisplayName": "Microsoft Graph",
"resourceId": "a3efc889-f1b7-4532-9e01-91e32d1039f4",
"scope": "user_impersonation User.Read"
},
{
"clientId": "b25985a8-9d2b-4fde-9fa7-6c4e43a8ddec",
"consentType": "Principal",
"id": "qIVZsiud3k-fp2xOQ6jd7InI76O38TJFngGR4y0QOfQ0hSwYE_R8SJGjet3IDjXV",
"principalId": "182c8534-f413-487c-91a3-7addc80e35d5",
"resourceDisplayName": "Microsoft Graph",
"resourceId": "a3efc889-f1b7-4532-9e01-91e32d1039f4",
"scope": "user_impersonation User.Read"
},
{
"clientId": "b25985a8-9d2b-4fde-9fa7-6c4e43a8ddec",
"consentType": "Principal",
"id": "qIVZsiud3k-fp2xOQ6jd7InI76O38TJFngGR4y0QOfTTWOHn3HzNR4glWFnXqytV",
"principalId": "e7e158d3-7cdc-47cd-8825-5859d7ab2b55",
"resourceDisplayName": "Microsoft Graph",
"resourceId": "a3efc889-f1b7-4532-9e01-91e32d1039f4",
"scope": "user_impersonation User.Read"
}
]
```

Based on [Update a delegated permission grant (oAuth2PermissionGrant)](https://docs.microsoft.com/en-us/graph/api/oauth2permissiongrant-update?view=graph-rest-1.0&tabs=http) API which **only supports updating `scope`** and the above observation, the primary key is the combination of all `clientId`, `consentType`, `principalId`, `resourceId`.

If the grant exists, the command fails with

```
Bad Request({"error":{"code":"Request_BadRequest","message":"Permission entry already exists.","innerError":{"date":"2022-01-13T09:57:53","request-id":"3e6a27ff-d875-4737-88d1-cd0a4ef092f7","client-request-id":"3e6a27ff-d875-4737-88d1-cd0a4ef092f7"}}})
```

## Solution

`az ad app permission grant` should not over-delete existing grants.