Add editorconfig (#1920)

Motivation:

Different projects use different indentation settings. We can configure
project specific settings in '.editorconfig' which is supported in
Xcode 16 beta.

Modifications:

- Add '.editorconfig' with settings for our project

Result:

Less friction when developing multiple projects with different editor
settings.

.editorconfig

@@ -0,0 +1,24 @@
+# Copyright 2024, gRPC Authors All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+root = true
+
+[*]
+end_of_line = lf
+trim_trailing_whitespace = true
+indent_style = space
+indent_size = 2
+
+[*.py]
+indent_size = 4