chore(stainless): add config for file header (#4126)

# What does this PR do?
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relevant issues if applicable. -->

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This PR adds Stainless config to specify the Meta copyright file header
for generated files.

Doing it via config instead of custom code will reduce the probability
of git conflict.

## Test Plan
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summaries. *Provide clear instructions so the plan can be easily
re-executed.* -->

- review preview builds

client-sdks/stainless/config.yml

@@ -463,6 +463,12 @@ resources:
 settings:
   license: MIT
   unwrap_response_fields: [data]
+  file_header: |
+    Copyright (c) Meta Platforms, Inc. and affiliates.
+    All rights reserved.
+
+    This source code is licensed under the terms described in the LICENSE file in
+    the root directory of this source tree.
 
 openapi:
   transformations: