feat: add post3 s3 proxy for postgresql

Signed-off-by: kjuulh <contact@kjuulh.io>

todos/POST3-005-xml-responses-extractors.md

@@ -0,0 +1,20 @@
+# POST3-005: XML response builders and extractors
+
+**Status:** Done
+**Priority:** P1
+**Blocked by:** POST3-004
+
+## Description
+
+Implement S3-compatible XML response serialization and request query parameter extraction.
+
+## Acceptance Criteria
+
+- [ ] `s3/responses.rs`:
+  - `list_buckets_xml(buckets)` — ListAllMyBucketsResult with Owner
+  - `list_objects_v2_xml(bucket, result, max_keys)` — ListBucketResult with Contents
+  - `error_xml(code, message, resource)` — S3 Error response
+- [ ] `s3/extractors.rs`:
+  - `ListObjectsQuery` — list-type, prefix, max-keys, continuation-token, start-after, delimiter
+- [ ] XML output matches S3 format (xmlns, element names, date format ISO 8601)
+- [ ] All responses include `x-amz-request-id` header (UUID)