base: add SmallVector

A vector with inline storage. Falls back on the
heap when exceeding it.

Bug: 205302474
Change-Id: Iaa479ad09dfe7e7c22e41e36e52d3ef1cc1c3706

BUILD

diff --git a/BUILD b/BUILD
index 5ce842e..b011148 100644
--- a/BUILD
+++ b/BUILD
@@ -355,6 +355,7 @@
         "include/perfetto/ext/base/pipe.h",
         "include/perfetto/ext/base/scoped_file.h",
         "include/perfetto/ext/base/small_set.h",
+        "include/perfetto/ext/base/small_vector.h",
         "include/perfetto/ext/base/string_splitter.h",
         "include/perfetto/ext/base/string_utils.h",
         "include/perfetto/ext/base/string_view.h",