Make signature-extension a string list

--signature-extension should be a string vector so that the user can provide multiple possible extensions to test for signatures.

I wrote this quickly, when I had a few minutes inbetween and the MR is incomplete.

Still pending:

• Documention
• In wget.c, use an inline function to add new jobs. Reduce code duplication
• When any one of the signature jobs completes successfully, abort the others.
• Run signature jobs only on a single thread, in the same job as the original one.

There might be something else that's pending. If you find something else that is missing, let me know,

Approver's checklist:

• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent with existing code
• This feature/change has adequate documentation added (if appropriate)
• No obvious mistakes / misspelling in the code