pw_kvs: Sectors abstraction

- Create the Sectors abstraction, which wraps the list of
  SectorDescriptors.
- Move sector-finding functionality to the Sectors class.
- Start tests for the Sectors class.

Change-Id: I2d804d14bba7c04880e578ade4da3488caec348c
diff --git a/pw_kvs/BUILD b/pw_kvs/BUILD
index f469cce..73a1b19 100644
--- a/pw_kvs/BUILD
+++ b/pw_kvs/BUILD
@@ -39,6 +39,7 @@
         "public/pw_kvs/internal/sectors.h",
         "public/pw_kvs/internal/span_traits.h",
         "pw_kvs_private/macros.h",
+        "sectors.cc",
     ],
     hdrs = [
         "public/pw_kvs/alignment.h",
@@ -174,6 +175,15 @@
     ],
 )
 
+pw_cc_test(
+    name = "sectors_test",
+    srcs = ["sectors_test.cc"],
+    deps = [
+        ":pw_kvs",
+        ":test_utils",
+    ],
+)
+
 # TODO: This binary is not building due to a linker error. The error does not occur in GN Builds.
 # A filegroup is used below so that the file is included in the Bazel build.
 # cc_binary(