rust: check time source for MsgGpsTime (#1375)

# Description

@swift-nav/devinfra

<!-- Changes proposed in this PR -->

# API compatibility

Does this change introduce a API compatibility risk?

<!-- Provide a short explanation why or why not -->

## API compatibility plan

If the above is "Yes", please detail the compatibility (or migration)
plan:

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# JIRA Reference

https://swift-nav.atlassian.net/browse/BOARD-XXXX

generator/sbpg/targets/rust.py

@@ -60,6 +60,11 @@
 BASE_TIME_MSGS = ["MSG_OBS", "MSG_OSR", "MSG_SSR"]
 
 CUSTOM_GPS_TIME_MSGS = {
+    "MSG_GPS_TIME": """
+if !matches!(self.time_source(), Ok(TimeSource::GnssSolution) | Ok(TimeSource::Propagated)) {
+    return None;
+}
+""".strip() + GPS_TIME,
     "MSG_IMU_RAW": """
 const IMU_RAW_TIME_STATUS_MASK: u32 = (1 << 30) | (1 << 31);
 if self.tow & IMU_RAW_TIME_STATUS_MASK != 0 {

rust/sbp/src/messages/navigation.rs

@@ -1888,6 +1888,12 @@ pub mod msg_gps_time {
 
         #[cfg(feature = "swiftnav")]
         fn gps_time(&self) -> Option<std::result::Result<time::MessageTime, time::GpsTimeError>> {
+            if !matches!(
+                self.time_source(),
+                Ok(TimeSource::GnssSolution) | Ok(TimeSource::Propagated)
+            ) {
+                return None;
+            }
             let tow_s = (self.tow as f64) / 1000.0;
             #[allow(clippy::useless_conversion)]
             let wn: i16 = match self.wn.try_into() {