Support the @encode TypeSpec decorator with "unixTimestamp"

[joseharriaga]

Consider the @encode decorator in TypeSpec used to specify that a utcDateTime property is encoded as a Unix timestamp:

model MyModel {
    @encode(DateTimeKnownEncoding.unixTimestamp, int32)
    myProperty: utcDateTime;    
}

The generated deserialization code for this property would look like this:

if (property.NameEquals("my_property"u8))
{
  myProperty = property.Value.GetDateTimeOffset("O");
  continue;
}

The problem is that Unix timestamps are represented as the number of seconds that have elapsed since the Unix epoch (January 1st, 1970, 00:00:00 UTC), but GetDateTimeOffset() expects a string. This causes the deserialization code above to fail.

More specifically, it seems like .NET's System.Text.Json does not offer first-class support for Unix timestamps; it defaults to ISO 8601-1:2019 instead (which is always a string; not a number). From the documentation, it seems that a custom converter is necessary:
🔗 https://learn.microsoft.com/en-us/dotnet/standard/datetime/system-text-json-support#using-unix-epoch-date-format