Make UiTokenAmount::ui_amount a String (#15447)

* Make UiTokenAmount::ui_amount a String

* Fixup solana-tokens

* Ignore spl downstream-project

storage-proto/proto/solana.storage.confirmed_block.rs

@@ -104,6 +104,8 @@ pub struct UiTokenAmount {
     pub decimals: u32,
     #[prost(string, tag = "3")]
     pub amount: ::prost::alloc::string::String,
+    #[prost(string, tag = "4")]
+    pub ui_amount_string: ::prost::alloc::string::String,
 }
 #[derive(Clone, PartialEq, ::prost::Message)]
 pub struct Reward {