Added source code for a filter for testing the release filters:

- Related Regolith PR: Bedrock-OSS/regolith#321

Author: Nusiq

reslease-filters-src/README.md

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+The filters defined in this directory are used for testing the [release filter feature](https://github.com/Bedrock-OSS/regolith/pull/321).

reslease-filters-src/hello-release-filter/README.md

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+This filter creates a `BP/hello_release_filter.txt` file and and writes the content of `data/hello-release-filter/message.txt` into it.
+
+The default message is "Hello from hello-release-filter!\n" (and it's stored in the filter's default data in the message.txt file).

reslease-filters-src/hello-release-filter/data/message.txt

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+Hello from hello-release-filter!

reslease-filters-src/hello-release-filter/filter.json

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+{
+	"filters": [
+		{
+			"runWith": "python",
+			"script": "./main.py"
+		}
+	]
+}

reslease-filters-src/hello-release-filter/main.py

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+from pathlib import Path
+
+def main():
+    with Path("data/hello-release-filter/message.txt").open(
+        "r", encoding="utf8"
+    ) as f:
+        message = f.read()
+    with Path("BP/hello_release_filter.txt").open("w", encoding="utf8") as f:
+        f.write(message)
+
+if __name__ == '__main__':
+    main()