Expose inner field of LSPSDateTime constructor

We require users to construct an `LSPSDateTime` and give it back to us
(e.g., in LSPS2 OpeningFeeParameters), but currently only offer
construction via `FromStr`. Here, we simply expose the wrapped
`DateTime` field, allowing users to construct an `LSPSDateTime`.

Author: tnull

lightning-liquidity/src/lsps0/ser.rs

@@ -195,7 +195,7 @@ pub struct LSPSRequestId(pub String);
 /// An object representing datetimes as described in bLIP-50 / LSPS0.
 #[derive(Clone, Debug, PartialEq, Eq, Hash, Deserialize, Serialize)]
 #[serde(transparent)]
-pub struct LSPSDateTime(chrono::DateTime<chrono::Utc>);
+pub struct LSPSDateTime(pub chrono::DateTime<chrono::Utc>);
 
 impl LSPSDateTime {
 	/// Returns the LSPSDateTime as RFC3339 formatted string.