Provide a global setting to set the default network offering for Differnet network types ( L2/Isolated/Shared)

### The required feature described as a wish

Currently, there is a global setting "cloud.kubernetes.cluster.network.offering" which is set to "DefaultNetworkOfferingforKubernetesService".

When a user executes the api (createKubernetesCluster) and doesn't provide a network, cloudstack uses the global setting value to automatically create a network based on the network offering

https://cloudstack.apache.org/api/apidocs-4.22/apis/createKubernetesCluster.html

Similarly, when a user executes the deployVirtualMachine api call , cloudstack uses the offerings mentioned in the sql query to launch vm based on the network offering

https://cloudstack.apache.org/api/apidocs-4.22/apis/deployVirtualMachine.html

deploy virtualmachine serviceofferingid=<> templateid=<> zoneid=<>

```
mysql> SELECT id, name, display_text, `default`, for_vpc FROM cloud.network_offerings WHERE availability = 'Required' AND system_only = 0;
+----+----------------------------------------------------+----------------------------------------------------------------+---------+---------+
| id | name | display_text | default | for_vpc |
+----+----------------------------------------------------+----------------------------------------------------------------+---------+---------+
| 10 | DefaultIsolatedNetworkOfferingWithSourceNatService | Offering for Isolated networks with Source Nat service enabled | 1 | 0 |
| 28 | DefaultNetworkOfferingforKubernetesService | Network Offering used for CloudStack Kubernetes service | 1 | 0 |
| 29 | DefaultNSXNetworkOfferingforKubernetesService | Network Offering for NSX CloudStack Kubernetes Service | 1 | 0 |
| 30 | DefaultNSXVPCNetworkOfferingforKubernetesService | Network Offering for NSX CloudStack Kubernetes service on VPC | 1 | 1 |
+----+----------------------------------------------------+----------------------------------------------------------------+---------+---------+
4 rows in set (0.01 sec)

```

Now if a Admin user wants to use differnet network offering as the default, he has to first mark all the offerings mentioned in the sql query as optional

Next, he has to make sure the newly created network offering as Required

```
mysql> SELECT id, name, display_text, `default`, for_vpc FROM cloud.network_offerings WHERE availability = 'Required' AND system_only = 0;
+----+------+--------------+---------+---------+
| id | name | display_text | default | for_vpc |
+----+------+--------------+---------+---------+
| 31 | test | test | 0 | 0 |
+----+------+--------------+---------+---------+
1 row in set (0.00 sec)

```

It's a tedious process and i think if CloudStack can have default global setting for each network type it would be beneficial from admin and

users who doesn't want to provide the network id during vm deployment and let cloudstack handle the network creation part

Default.isolated.network.offering
Default.shared.network.offering
Default.l2.network.offering
Default.vpc.network.offering
Default.shared.network.offering
Default.nsx.network.offering
Default.netris.network.offering

Ref

https://github.com/apache/cloudstack/issues/12116#issuecomment-3576829083

从 createKubernetesCluster 和 deployVirtualMachine API 路径、现有的 cloud.kubernetes.cluster.network.offering 设置，以及 issue 中所示的 cloud.network_offerings 查询开始。确定 isolated、shared、L2、VPC、NSX 和 Netris 网络的默认值如何选择。完成的标准是：管理员可以配置不同的全局默认值，而无需手动更改 offering 的可用性，并且两个 API 流程都使用相应的已配置默认值。

由索引模型根据 Issue 内容生成。