Serialization of `VaultPositionType`

[larry0x]

NOTE: This is a state- and API-breaking change!

The VaultPositionType is defined as

#[cw_serde]
pub enum VaultPositionType {
    UNLOCKED,
    LOCKED,
    UNLOCKING,
}

However, all caps names don't work well with JSON serialization. For example, VaultPositionType::UNLOCKED is serialized to JSON string "u_n_l_o_c_k_e_d". Imo, this is undesirable.

To fix this, we can either no longer naming the variants in all caps:

#[cw_serde]
pub enum VaultPositionType {
    Unlocked,
    Locked,
    Unlocking,
}

or use #[serde(rename)]:

#[cw_serde]
pub enum VaultPositionType {
    #[serde(rename = "unlocked")]
    UNLOCKED,
    #[serde(rename = "locked")]
    LOCKED,
    #[serde(rename = "unlocking")]
    UNLOCKING,
}