Allow user to configure automatic suggestion services

### Describe the problem

For the last few days, "hosted" weblate has been lacking suggestions from all translator sites (except LibreTranslate).

I would say the admin has disabled them, but this could, very well, be the exhaustion of monthly free usage.

Whatever the reason, it definitely slows down me for the thousand-strings project I am working on.

### Describe the solution you'd like

You can never know what may happen to the weblate version ("hosted" or custom) and the suggestions.

They are definitely great tools to ease the pain of especially the writing, and provide speed to the users.

This feature should not be a responsibility of Weblate instance, and must be "shareable" to users at will.

So, I would like to have a way to use my own API keys for those sites (whether free or paid) in such a situation.

### Describe alternatives you've considered

The alternative is to copy-paste strings into a translator site and then copy-paste the result back into Weblate. It is too much tiresome.

### Screenshots

_No response_

### Additional context

_No response_