Ability to add MCP custom server additional customisations

### Feature summary

Add more customisation options for custom MCP servers

### What problem are you trying to solve?

Unable to add custom configuration required for some MCP servers, only variables via the UI.

e.g. nested configuration cannot be added.

```json
"auth": {
"redirectPort": 12798
}
```

### Proposed solution

add addition options through the UI to be able to support additional configuration for some MCP servers,

e.g. workiq preview MCP server requires this config currently.

```json
{
"mcpServers": {
"workiq-preview": {
"type": "http",
"url": "https://workiq.svc.cloud.microsoft/mcp",
"oauthClientId": "ba081686-5d24-4bc6-a0d6-d034ecffed87",
"oauthPublicClient": true,
"auth": {
"redirectPort": 12798
}
}
}
}
```

### Workflow impact

All users affected.

Dont have to edit the mcpconfig.json file in global copilot settings directly and can add complete MCP custom servers via the app

### Installation context

_No response_

### Additional context

_No response_

该 issue 没有指定源文件或测试。首先跟踪自定义 MCP 服务器配置 UI 及其对 mcpconfig.json 的处理，并将当前仅支持变量的流程与嵌套 auth 示例进行比较；当用户可以通过应用添加、保存和重新加载该配置，而无需直接编辑文件时，即视为完成。

由索引模型根据 Issue 内容生成。