performance-metrics: Add AIO completion drain micro benchmark

Add micro_block_raw_aio_drain_128_us and
micro_block_raw_aio_drain_256_us tests that submit N AIO writes
to a temporary file, wait for the eventfd signal, then time how
long it takes to drain all completions via next_completed_request().

This measures per completion syscall overhead and provides a
baseline before any batching optimizations.

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>

performance-metrics/Cargo.toml

@@ -5,8 +5,10 @@ name = "performance-metrics"
 version = "0.1.0"
 
 [dependencies]
+block = { path = "../block" }
 clap = { workspace = true, features = ["wrap_help"] }
 dirs = { workspace = true }
+libc = { workspace = true }
 serde = { workspace = true, features = ["derive", "rc"] }
 serde_json = { workspace = true }
 test_infra = { path = "../test_infra" }