Feature Request: Allow fillcharhl to be a table of two elements (or a function)

[Nawar9]

It would be nice to have the ability to define two highlight groups for the fillchar of signs: one for the focused line ('cursorline' with v:relnum == 0) and another for non-focused lines.

Example:

A potential API could allow defining highlight groups as follows:

fillcharhl = { "GitSignsFillChar", "GitSignsFillCharCursorLine"}

Another option:

fillcharhl = function(args)
    return (args.cul and args.relnum == 0) and  "GitSignsFillCharCursorLine" or "GitSignsFillChar"
end

Note: The same functionality could also apply to the hl key of segment