feat(lsp): include original request params in handler ctx

This is mostly motivated by https://github.com/neovim/neovim/issues/12326

Client side commands might need to access the original request
parameters.

Currently this is already possible by using closures with
`vim.lsp.buf_request`, but the global handlers so far couldn't access
the request parameters.

runtime/doc/lsp.txt

@@ -224,6 +224,11 @@ For |lsp-request|, each |lsp-handler| has this signature: >
                                             The ID of the |vim.lsp.client|.
                             {bufnr}     (Buffer)
                                            Buffer handle, or 0 for current.
+
+                            {params}    (table|nil)
+                                           The parameters used in the original request
+                                           which resulted in this handler
+                                           call.
             {config}    (table)
                             Configuration for the handler.
 
@@ -234,6 +239,7 @@ For |lsp-request|, each |lsp-handler| has this signature: >
                             To configure a particular |lsp-handler|, see:
                                 |lsp-handler-configuration|
 
+
         Returns: ~
             The |lsp-handler| can respond by returning two values: `result, err`
               Where `err` must be shaped like an RPC error:

runtime/lua/vim/lsp.lua

@@ -896,7 +896,7 @@ function lsp.start_client(config)
 
     local _ = log.debug() and log.debug(log_prefix, "client.request", client_id, method, params, handler, bufnr)
     return rpc.request(method, params, function(err, result)
-      handler(err, result, {method=method, client_id=client_id, bufnr=bufnr})
+      handler(err, result, {method=method, client_id=client_id, bufnr=bufnr, params=params})
     end)
   end
 

test/functional/plugin/lsp_spec.lua

@@ -2353,6 +2353,10 @@ describe('LSP', function()
           eq(0, signal, "exit signal", fake_lsp_logfile)
         end;
         on_handler = function(err, result, ctx)
+          -- Don't compare & assert params, they're not relevant for the testcase
+          -- This allows us to be lazy and avoid declaring them
+          ctx.params = nil
+
           eq(table.remove(test.expected_handlers), {err, result, ctx}, "expected handler")
           if ctx.method == 'start' then
             exec_lua("vim.lsp.buf.rename()")