Allow custom floating window style

[SheffeyG]

This change allow users to custom floating window like #685, so we can configure it as:

floating = {
    width = 0.5, -- percentage
    height = 30, -- row
    border = "single",
    ...
}

[SheffeyG]

@CKolkey There're some type warnings here, and I am not familiar with Lua's type system, I can resolve the type warnings by this:

modified lua/neogit/lib/buffer.lua
@@ -317,7 +317,7 @@ function Buffer:show()
     elseif self.kind == "vsplit_left" then
       win = api.nvim_open_win(self.handle, true, { split = "left", vertical = true })
     elseif self.kind == "floating" then
-      local floating = require("neogit.config").values.floating
+      local floating = require("neogit.config").values.floating or {}
 
       -- Creates the border window
       local vim_height = vim.o.lines
@@ -330,7 +330,7 @@ function Buffer:show()
       floating.row = vim_height * 0.15
       floating.focusable = true
 
-      local content_window = api.nvim_open_win(self.handle, true, floating)
+      local content_window = api.nvim_open_win(self.handle, true, vim.tbl_deep_extend("force", {}, floating))
 
       api.nvim_win_set_cursor(content_window, { 1, 0 })
       win = content_window

but in this way is kind bad since my method of reading custom configurations might be wrong, could you tell me the correct way to do it?