[Feature]: Set custom User-agent for fetching subscriptions

Is your feature request related to a problem?

Some VPN Service providers blocking subscription requests from unsupported clients. Reply for that request is "This client is not supported, use HAPP, INCY", etc

Describe the solution you'd like

Please add field for manual changing User-Agent in subscription request, like this:

It's good to customize other fields of request, like x-hwid, x-device-os, x-ver-os, x-device-model, etc.

Alternatives you've considered

No response

Concrete use case

No response

Mockups, screenshots, or examples

No response

Which parts of the panel would this affect?

Frontend (UI / panel pages), Backend (API endpoints, login, settings)

Are you willing to help?

• I'd like to implement this feature myself and open a PR.
• I can help test once a PR is open.

Before submitting

• I searched existing issues and this feature has not been requested.
• This feature request is written in English.