sstables: add error injection point in write_components

Add a `write_components_writer_created` error injection point in
`sstable::write_components()` between writer creation and fragment
consumption.

This injection is needed by the out-of-space streaming test (added in
the next patch) to reliably pause SSTable writing at the right moment:
after the SSTable writer has been created and files exist on disk, but
before mutation fragments are consumed.

Pausing earlier (before writer creation) would not work because there
are no files on disk yet, while pausing later (after consuming fragments)
would be too late to reliably push the node into critical disk utilization.

sstables/sstables.cc

@@ -2531,6 +2531,7 @@ future<> sstable::write_components(
     return seastar::async([this, mr = std::move(mr), estimated_partitions, schema = std::move(schema), cfg, stats] () mutable {
         auto close_mr = deferred_close(mr);
         auto wr = get_writer(*schema, estimated_partitions, cfg, stats);
+        utils::get_local_injector().inject("write_components_writer_created", utils::wait_for_message(std::chrono::seconds(30))).get();
         mr.consume_in_thread(std::move(wr));
     }).finally([this] {
         assert_large_data_handler_is_running();