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_

The issue names no source file or test. Start by tracing the custom MCP server configuration UI and its handling of mcpconfig.json, comparing the current variable-only flow with the nested auth example; done means users can add, save, and reload that configuration through the app without editing the file directly.

Written by the indexing model from the issue text.