feat: Support a custom config

[JRasmusBm]

Why is the change needed?

I use an external library for some dap operations, and it's nice to be able to execute it from the list of configurations.

My usage:

https://github.com/JRasmusBm/dotfiles/blob/e62f7027c07e3d71a849e0edad4967d2bb0ee05f/vim/lua/jrasmusbm/dap/python.lua#L30-L35

dap.configurations.python = {
  -- ...
  {
    type = "python",
    request = "custom",
    name = "Attach to debugger",
    command = "Debugpy attach 0.0.0.0 5678"
  },
 -- ...
}