TimeSpan-Style TimeDuration Constructors in C# Bindings (#2778)

# Description of Changes

This change adds the following `System.TimeSpan`-style static
construction methods to `SpacetimeDB.TimeDuration`:
* `static TimeDuration FromMilliseconds(double milliseconds)`
* `static TimeDuration FromSeconds(double seconds)`
* `static TimeDuration FromMinutes(double minutes)`
* `static TimeDuration FromHours(double hours)`
* `static TimeDuration FromDays(double days)`

These mirror the equivalently named static methods on `System.TimeSpan`
and dramatically improve usability and familiarity for experienced C#
users with no more overhead than the user performing the multiplication
themselves.

Wish I'd thought to do this before v1.1.2 got released. Ah well.

# API and ABI breaking changes

None. Convenience methods added in bindings only.

# Expected complexity level and risk

1 (potentially up to a low 2 if cleanup is desired elsewhere in the
bindings to leverage these new methods).

# Testing

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reviewers to do,
so that you're confident that all the changes work as expected! -->

- [x] Ensure the changes build. <!-- maybe a test you want to do -->
- [ ] New contributor check! Review to make sure repo style & substance
standards are complied with. <!-- maybe a test you want a reviewer to
do, so they can check it off when they're satisfied. -->

Author: wes-sleeman

crates/bindings-csharp/BSATN.Runtime/Builtins.cs

@@ -432,6 +432,33 @@ public record struct TimeDuration(long Microseconds) : IStructuralReadWrite
 {
     public static readonly TimeDuration ZERO = new(0);
 
+    /// <summary>
+    /// Returns a <see cref="TimeDuration"/> that represents a specified number of <paramref name="milliseconds"/>, accurate to the nearest microsecond.
+    /// </summary>
+    public static TimeDuration FromMilliseconds(double milliseconds) =>
+        new((long)(milliseconds * 1000L));
+
+    /// <summary>
+    /// Returns a <see cref="TimeDuration"/> that represents a specified number of <paramref name="seconds"/>, accurate to the nearest microsecond.
+    /// </summary>
+    public static TimeDuration FromSeconds(double seconds) =>
+        new((long)(seconds * Util.MicrosecondsPerSecond));
+
+    /// <summary>
+    /// Returns a <see cref="TimeDuration"/> that represents a specified number of 60-second <paramref name="minutes"/>.
+    /// </summary>
+    public static TimeDuration FromMinutes(double minutes) => FromSeconds(minutes * 60);
+
+    /// <summary>
+    /// Returns a <see cref="TimeDuration"/> that represents a specified number of 60-minute <paramref name="hours"/>.
+    /// </summary>
+    public static TimeDuration FromHours(double hours) => FromMinutes(hours * 60);
+
+    /// <summary>
+    /// Returns a <see cref="TimeDuration"/> that represents a specified number of 24-hour <paramref name="days"/>.
+    /// </summary>
+    public static TimeDuration FromDays(double days) => FromHours(days * 24);
+
     public static implicit operator TimeSpan(TimeDuration d) =>
         new(d.Microseconds * Util.TicksPerMicrosecond);