Replace Expiration by MillisecondsExpiration in RegisterMerkleRoot

[daniel-wehbe]

Description

We recently have been changing the instances of Expiration to use our Milliseconds structs. Found an instance that was missed:

    RegisterMerkleRoot {
        /// MerkleRoot is hex-encoded merkle root.
        merkle_root: String,
        expiration: Option<Expiration>,
        total_amount: Option<Uint128>,
    },

Acceptance Criteria

• The Expiration replaced by MillisecondsExpiration
• Make modifications where needed

[daniel-wehbe]

Can we aslo replace the Bid structure in the Auction to use Milliseconds instead of TimeStamp:

pub struct BidsResponse {
    pub bids: Vec<Bid>,
}

pub struct Bid {
    pub bidder: String,
    pub amount: Uint128,
    pub timestamp: Timestamp,
}