Add optional image_hash field to CHIP-420 for resource integrity

Also, add reference to Solana Metaplex Token Metadata.

Author: imikushin

CHIPs/CHIP-0420/README.md

@@ -7,7 +7,9 @@ Authors:
 Created: 2025-02-25
 ---
 
-This proposal borrows ideas from Cardano CIP-0068 Datum Metadata Standard [1].
+# CHIP-420. Token Metadata
+
+This proposal borrows ideas from Cardano CIP-0068 Datum Metadata Standard [1] and Solana Metaplex Token Metadata [2].
 
 A fungible token value in Charms is represented by a single unsigned integer. This is great as a compact representation
 of the token amount. A fungible token, as an asset, is fully defined by its App structure:
@@ -18,6 +20,8 @@ t/{app_identity}/{app_vk}
 
 If we replace the tag character `t` with `n`, we will have a full identifier for the fungible token's reference NFT.
 
+## Proposal
+
 We propose the following structure for reference NFT data:
 
 ```yaml
@@ -46,6 +50,12 @@ url: ?<URI>
 # Should point to a resource with media type `image/png`, `image/jpeg` or `image/svg+xml`.
 image: ?<URI>
 
+# (optional)
+# SHA-256 hash of the remote resource that `image` is pointing to.
+# When not present, the resource is assumed to be mutable (which is 100% okay for a home page of a website),
+# unless `image` is a `data` URI.
+image_hash: ?<hex_string>
+
 # (optional) 
 # Number of digits after decimal point in the smallest denomination of the token.
 # A token amount in Charms is always a natural number quantity of its smallest denomination. 
@@ -69,3 +79,5 @@ minimum (recommended, but not required: all fields are optional).
 ### References
 
 [1] Cardano CIP-0068 Datum Metadata Standard. https://github.com/cardano-foundation/CIPs/tree/master/CIP-0068
+
+[2] Solana Metaplex Token Metadata. https://developers.metaplex.com/token-metadata