rest: allow reading partial block data from storage

It will allow fetching specific transactions using an external index,
following https://github.com/bitcoin/bitcoin/pull/32541#issuecomment-3267485313.

Co-authored-by: Hodlinator <172445034+hodlinator@users.noreply.github.com>
Co-authored-by: Lőrinc <pap.lorinc@gmail.com>

doc/release-notes-33657.md

@@ -0,0 +1,5 @@
+New REST API
+------------
+
+- A new REST API endpoint (`/rest/blockpart/BLOCKHASH.bin?offset=X&size=Y`) has been introduced
+  for efficiently fetching a range of bytes from block `BLOCKHASH`.