Upload Test Remote Extensions (#4792)

We need some real extensions in S3 to accurately test the code for
handling remote extensions.
In this PR we just upload three extensions (anon, kq_imcx and postgis), which is
enough for testing purposes for now. In addition to creating and
uploading the extension archives, we must generate a file
`ext_index.json` which specifies important metadata about the
extensions.

---------

Co-authored-by: Anastasia Lubennikova <anastasia@neon.tech>
Co-authored-by: Alexander Bayandin <alexander@neon.tech>

scripts/combine_control_files.py

@@ -0,0 +1,33 @@
+#! /usr/bin/env python3
+# Script to generate ext_index.json metadata file
+# that stores content of the control files and location of extension archives
+# for all extensions in extensions subdir.
+import argparse
+import json
+from pathlib import Path
+
+if __name__ == "__main__":
+    parser = argparse.ArgumentParser(description="generate ext_index.json")
+    parser.add_argument("pg_version", type=str, choices=["v14", "v15"], help="pg_version")
+    parser.add_argument("BUILD_TAG", type=str, help="BUILD_TAG for this compute image")
+    args = parser.parse_args()
+    pg_version = args.pg_version
+    BUILD_TAG = args.BUILD_TAG
+
+    ext_index = {}
+    EXT_PATH = Path("extensions")
+    for extension in EXT_PATH.iterdir():
+        if extension.is_dir():
+            control_data = {}
+            for control_file in extension.glob("*.control"):
+                if control_file.suffix != ".control":
+                    continue
+                with open(control_file, "r") as f:
+                    control_data[control_file.name] = f.read()
+            ext_index[extension.name] = {
+                "control_data": control_data,
+                "archive_path": f"{BUILD_TAG}/{pg_version}/extensions/{extension.name}.tar.zst",
+            }
+
+    with open("ext_index.json", "w") as f:
+        json.dump(ext_index, f)