pw_kvs: Add value_type to Key

Some std::span implementations rely on the value_type for the
containers. This CL adds proper value_type for the Key.

Change-Id: I147074deafb26bf928aad821e4dee2c58a7b2f22
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/99281
Reviewed-by: Wyatt Hepler <hepler@google.com>
Pigweed-Auto-Submit: Dennis Kormalev <denk@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/pw_kvs/public/pw_kvs/key.h b/pw_kvs/public/pw_kvs/key.h
index 06f06c0..bf9a428 100644
--- a/pw_kvs/public/pw_kvs/key.h
+++ b/pw_kvs/public/pw_kvs/key.h
@@ -28,6 +28,8 @@
 // platforms without C++17.
 class Key {
  public:
+  using value_type = const char;
+
   // Constructors
   constexpr Key() : str_{nullptr}, length_{0} {}
   constexpr Key(const Key&) = default;