pw_kvs: Move EntryHeaderFormat to pw_kvs/format.h

- Move EntryHeaderFormat and the EntryHeader struct to pw_kvs/format.h.
- Rename EntryHeaderFormat to EntryFormat.

Change-Id: Ic5e53c688cd3a483101c7985def6e4bef9c3cbaf
diff --git a/pw_kvs/BUILD b/pw_kvs/BUILD
index 64c44b3..0bb2120 100644
--- a/pw_kvs/BUILD
+++ b/pw_kvs/BUILD
@@ -41,6 +41,7 @@
         "public/pw_kvs/checksum.h",
         "public/pw_kvs/crc16_checksum.h",
         "public/pw_kvs/flash_memory.h",
+        "public/pw_kvs/format.h",
         "public/pw_kvs/key_value_store.h",
         "public/pw_kvs/output.h",
     ],