Add displayDecimals to the currency object.

This helps the sender display the receiving currency amount correctly in their UI.

Protocol: uma-universal-money-address/protocol#7

src/currency.rs

@@ -2,13 +2,34 @@ use serde::{Deserialize, Serialize};
 
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
 pub struct Currency {
+    // The ISO 4217 currency code of the currency (eg. USD).
     pub code: String,
+
+    // The full display name of the currency (eg. US Dollars).
     pub name: String,
+
+    // The symbol of the currency (eg. $ for USD).
     pub symbol: String,
+
+    // The estimated millisatoshis per smallest "unit" of this currency (eg. 1 cent in USD).
     #[serde(rename = "multiplier")]
     pub millisatoshi_per_unit: i64,
+
+    // The minimum amount of the currency that can be sent in a single transaction. This is in the
+    // smallest unit of the currency (eg. cents for USD).
     #[serde(rename = "minSendable")]
     pub min_sendable: i64,
+
+    // The maximum amount of the currency that can be sent in a single transaction. This is in the
+    // smallest unit of the currency (eg. cents for USD).
     #[serde(rename = "maxSendable")]
     pub max_sendable: i64,
+
+    // The number of digits after the decimal point for display on the sender side. For example,
+    // in USD, by convention, there are 2 digits for cents - $5.95. in this case, `displayDecimals`
+    // would be 2. Note that the multiplier is still always in the smallest unit (cents). This field
+    // is only for display purposes. The sender should assume zero if this field is omitted, unless
+    // they know the proper display format of the target currency.
+    #[serde(rename = "displayDecimals")]
+    pub display_decimals: Option<i64>,
 }

src/uma_test.rs

@@ -148,6 +148,7 @@ mod tests {
             millisatoshi_per_unit: 34150,
             min_sendable: 1,
             max_sendable: 10000000,
+            display_decimals: Some(2),
         }];
 
         let response = get_lnurlp_response(