derive Serialize for GovernanceParameters

this would enable @fadroma/namada to automatically serialize the response from `/vp/governance/parameters` to JSON without having to explicitly list each field (see https://github.com/hackbg/fadroma/blob/e7d5d551f818a6441ef54309a2517792b03ee7d3/packages/namada/src/decode.rs#L201-L223)

crates/governance/src/parameters.rs

@@ -1,3 +1,4 @@
+use serde::Serialize;
 use namada_core::borsh::{BorshDeserialize, BorshSerialize};
 use namada_core::token;
 use namada_macros::BorshDeserializer;
@@ -18,6 +19,7 @@ use super::storage::keys as goverance_storage;
     BorshSerialize,
     BorshDeserialize,
     BorshDeserializer,
+    Serialize,
 )]
 /// Governance parameter structure
 pub struct GovernanceParameters {