[Feature Request] Support Microsoft Graph Delete oAuth2PermissionGrant API

## Context

MS Graph API has [Delete oAuth2PermissionGrant (a delegated permission grant)](https://docs.microsoft.com/en-us/graph/api/oauth2permissiongrant-delete?view=graph-rest-1.0&tabs=http), but there is no corresponding Azure CLI command.

```
> az ad app permission -h

Group
az ad app permission : Manage an application's OAuth2 permissions.

Commands:
add : Add an API permission.
admin-consent : Grant Application & Delegated permissions through admin-consent.
delete : Remove an API permission.
grant : Grant the app an API Delegated permissions.
list : List API permissions the application has requested.
list-grants : List Oauth2 permission grants.
```

## Solution

- Introduce a corresponding command like `az ad app permission delete-grant`, or
- Move all `grant`-related commands under one `az ad app permission grant` command group, but this is certainly a big breaking change:
- `az ad app permission grant create`
- `az ad app permission grant list`
- `az ad app permission grant delete`