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/REST-interface.md

@@ -47,6 +47,11 @@ The HTTP request and response are both handled entirely in-memory.
 
 With the /notxdetails/ option JSON response will only contain the transaction hash instead of the complete transaction details. The option only affects the JSON response.
 
+- `GET /rest/blockpart/<BLOCK-HASH>.<bin|hex>?offset=<OFFSET>&size=<SIZE>`
+
+Given a block hash: returns a block part, in binary or hex-encoded binary formats.
+Responds with 404 if the block or the byte range doesn't exist.
+
 #### Blockheaders
 `GET /rest/headers/<BLOCK-HASH>.<bin|hex|json>?count=<COUNT=5>`
 

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`.