utils::seekable_source: Add a seekable IO source type

Extension of data_source, with the ability to
a.) Seek in any direction, i.e. move backwards. Thus not pure stream.
b.) Read a limited number of bytes.

The very transparent reason for the interface is to have a base
abstraction for providing a read-only file layer for networked
resources.

utils/seekable_source.hh

@@ -0,0 +1,59 @@
+/*
+ * Copyright (C) 2025-present ScyllaDB
+ */
+
+/*
+ * SPDX-License-Identifier: LicenseRef-ScyllaDB-Source-Available-1.0
+ */
+
+#pragma once
+
+#include <chrono>
+#include <seastar/core/iostream.hh>
+
+class seekable_data_source_impl : public seastar::data_source_impl {
+public:
+    virtual seastar::future<seastar::temporary_buffer<char>> get(size_t limit) = 0;
+    virtual seastar::future<> seek(uint64_t pos) = 0;
+    virtual seastar::future<uint64_t> size() {
+        return seastar::make_ready_future<uint64_t>(0);
+    }
+    virtual seastar::future<std::chrono::system_clock::time_point> timestamp() {
+        return seastar::make_ready_future<std::chrono::system_clock::time_point>();
+    }
+};
+
+class seekable_data_source : public seastar::data_source {
+public:
+    seekable_data_source(std::unique_ptr<seekable_data_source_impl> src)
+        : seastar::data_source(std::move(src))
+    {}
+    seastar::future<seastar::temporary_buffer<char>> get(size_t limit) noexcept {
+        try {
+            return static_cast<seekable_data_source_impl*>(impl())->get(limit);
+        } catch (...) {
+            return seastar::current_exception_as_future<seastar::temporary_buffer<char>>();
+        }
+    }
+    seastar::future<> seek(uint64_t pos) noexcept {
+        try {
+            return static_cast<seekable_data_source_impl*>(impl())->seek(pos);
+        } catch (...) {
+            return seastar::current_exception_as_future<>();
+        }
+    }
+    seastar::future<uint64_t> size() noexcept {
+        try {
+            return static_cast<seekable_data_source_impl*>(impl())->size();
+        } catch (...) {
+            return seastar::current_exception_as_future<uint64_t>();
+        }
+    }
+    seastar::future<std::chrono::system_clock::time_point> timestamp() noexcept {
+        try {
+            return static_cast<seekable_data_source_impl*>(impl())->timestamp();
+        } catch (...) {
+            return seastar::current_exception_as_future<std::chrono::system_clock::time_point>();
+        }
+    }
+};