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 = {}

test/functional/plugin/lsp_spec.lua

@@ -6490,7 +6490,7 @@ describe('LSP', function()
       )
     end)
 
-    it('supports async function for root_dir', function()
+    it('async root_dir, cmd(…,config) gets resolved config', function()
       exec_lua(create_server_definition)
 
       local tmp1 = t.tmpname(true)
@@ -6504,7 +6504,10 @@ describe('LSP', function()
         })
 
         vim.lsp.config('foo', {
-          cmd = server.cmd,
+          cmd = function(dispatchers, config)
+            _G.test_resolved_root = config.root_dir --[[@type string]]
+            return server.cmd(dispatchers, config)
+          end,
           filetypes = { 'foo' },
           root_dir = function(bufnr, cb)
             assert(tmp1 == vim.api.nvim_buf_get_name(bufnr))
@@ -6527,6 +6530,12 @@ describe('LSP', function()
           end)
         )
       end)
+      eq(
+        'some_dir',
+        exec_lua(function()
+          return _G.test_resolved_root
+        end)
+      )
     end)
 
     it('starts correct LSP and stops incorrect LSP when filetype changes', function()
@@ -6784,10 +6793,8 @@ describe('LSP', function()
       markers_resolve_to({ 'marker_a', { 'marker_b', 'marker_d' } }, tmp_root)
       markers_resolve_to({ 'foo', { 'bar', 'baz' }, 'marker_d' }, dir_b)
     end)
-  end)
 
-  describe('vim.lsp.is_enabled()', function()
-    it('works', function()
+    it('vim.lsp.is_enabled()', function()
       exec_lua(function()
         vim.lsp.config('foo', {
           cmd = { 'foo' },