backport: feat(lsp): pass resolved config to cmd() #34560

Problem:
In LSP configs, the function form of `cmd()` cannot easily get the
resolved root dir (workspace). One of the main use-cases of a dynamic
`cmd()` is to be able to start a new server  whose binary may be located
*in the workspace* ([example](https://github.com/neovim/nvim-lspconfig/pull/3912)).

Compare `reuse_client()`, which also receives the resolved config.

Solution:
Pass the resolved config to `cmd()`.


(cherry picked from commit 32f30c4874)

Co-authored-by: Julian Visser <12615757+justmejulian@users.noreply.github.com>

test/functional/plugin/lsp/testutil.lua

@@ -54,7 +54,7 @@ M.create_server_definition = function()
     local server = {}
     server.messages = {}
 
-    function server.cmd(dispatchers)
+    function server.cmd(dispatchers, _config)
       local closing = false
       local handlers = opts.handlers or {}
       local srv = {}