sstables/sstable: expose v2 variant of write_components()

In parallel to the existing v1 one. In the next patches we start
migrating users to the v2 variant incrementally and finally remove the
v1 variant.

sstables/sstables.cc

@@ -1673,6 +1673,23 @@ future<> sstable::write_components(
     });
 }
 
+future<> sstable::write_components(
+        flat_mutation_reader_v2 mr,
+        uint64_t estimated_partitions,
+        schema_ptr schema,
+        const sstable_writer_config& cfg,
+        encoding_stats stats,
+        const io_priority_class& pc) {
+    assert_large_data_handler_is_running();
+    return seastar::async([this, mr = std::move(mr), estimated_partitions, schema = std::move(schema), cfg, stats, &pc] () mutable {
+        auto close_mr = deferred_close(mr);
+        auto wr = get_writer(*schema, estimated_partitions, cfg, stats, pc);
+        mr.consume_in_thread(std::move(wr));
+    }).finally([this] {
+        assert_large_data_handler_is_running();
+    });
+}
+
 future<> sstable::generate_summary(const io_priority_class& pc) {
     if (_components->summary) {
         co_return;

sstables/sstables.hh

@@ -70,6 +70,7 @@ namespace fs = std::filesystem;
 extern logging::logger sstlog;
 class key;
 class sstable_writer;
+class sstable_writer_v2;
 class sstables_manager;
 class metadata_collector;
 
@@ -259,6 +260,13 @@ public:
             encoding_stats stats,
             const io_priority_class& pc = default_priority_class());
 
+    future<> write_components(flat_mutation_reader_v2 mr,
+            uint64_t estimated_partitions,
+            schema_ptr schema,
+            const sstable_writer_config&,
+            encoding_stats stats,
+            const io_priority_class& pc = default_priority_class());
+
     sstable_writer get_writer(const schema& s,
         uint64_t estimated_partitions,
         const sstable_writer_config&,