derive Serialize for PosParams and OwnedPosParams

this would enable our client library `@fadroma/namada` to automatically convert the response from `/vp/pos/pos_params` without having to explicitly list every field (see https://github.com/hackbg/fadroma/blob/e7d5d551f818a6441ef54309a2517792b03ee7d3/packages/namada/src/decode.rs#L201-L223)

crates/proof_of_stake/src/parameters.rs

@@ -3,6 +3,7 @@
 use std::str::FromStr;
 
 use borsh::{BorshDeserialize, BorshSerialize};
+use serde::{Serialize, Deserialize};
 use namada_core::arith::checked;
 use namada_core::dec::Dec;
 use namada_core::storage::Epoch;
@@ -17,7 +18,7 @@ use thiserror::Error;
 
 /// Proof-of-Stake system parameters. This includes parameters that are used in
 /// PoS but are read from other accounts storage (governance).
-#[derive(Debug, Clone, BorshDeserialize, BorshDeserializer, BorshSerialize)]
+#[derive(Debug, Clone, BorshDeserialize, BorshDeserializer, BorshSerialize, Serialize)]
 pub struct PosParams {
     /// PoS-owned params
     pub owned: OwnedPosParams,
@@ -28,7 +29,7 @@ pub struct PosParams {
 
 /// Proof-of-Stake system parameters owned by the PoS address, set at genesis
 /// and can only be changed via governance
-#[derive(Debug, Clone, BorshDeserialize, BorshDeserializer, BorshSerialize)]
+#[derive(Debug, Clone, BorshDeserialize, BorshDeserializer, BorshSerialize, Serialize)]
 pub struct OwnedPosParams {
     /// A maximum number of consensus validators
     pub max_validator_slots: u64,