Add execute message to provide liquidity at user specified price in the Astroport Lper

[uditvira]

We should modify the LPer to incorporate user-provided price information when providing liquidity to PCL pools. This can be done by separating the one provide_double_sided_liquidity message into two. One that provides liquidity at the pool ratio and one that provides liquidity at any ratio supplied by the user.

#[derive(Serialize, Deserialize, Clone, Debug, PartialEq, JsonSchema)]
#[serde(rename_all = "snake_case")]
pub enum ExecuteMsg {
    // Provides liquidity at the pool ratio and optionally validates that the ratio is within the expected range. Typically used for XYK
    ProvideLiquidityAtPoolRatio {
        expected_pool_ratio_range: Option<DecimalRange>,
    },

    // Provides liquidity at user supplied ratio. Does not validate pool ratio. Typically used for custom pool types, like PCL
    ProvideLiquidityAtUserRatio {
        price_ratio: Decimal, // User-specified ratio
    },
    // ... other execute messages ...
}

Additionally, in the documentation, we should mention that the ProvideLiquidityAtUserRatio message should be used for PCL pools and the ProvideLiquidityAtPoolRatio for XYK pools.