Fixes #2866

When using default NodaTime serialization, the Instant is serialized using the ExtendedIso (uuuu'-'MM'-'dd'T'HH':'mm':'ss;FFFFFFFFF'Z') format and is saved as such to the data jsonb column.

But when the data is read and deserialized the Instant format depends on how the data is read:

1. When the Instant is read as a part of a parent object e.g. session.Query<ParentObj>().SingleAsync(p => p.Id = fooId) then the jsonb is read as is and the Instant property is formatted as an ExtendedIso string - not an issue
2. But when the Instant is read by using LINQ Select to project only specific properties from the data column (e.g. session.Query<ParentObj>().Select(p => new { p.Id, p.InstantProp }).SingleAsync()), in the SQL SELECT statement, the properties are cast to the Npgsql type the property type maps to.
   Instant maps to timestamp with time zone and when the cast is performed, the format contains the +00:00 offset at the end instead of the Z, so the deserialization attempt with the ExtendedIso format fails

I've added the InstantJsonConverter for both STJ and Newtonsoft which tries the alternate format deserialization if the ExtendedIso deserialization fails, and have registered it as the Instant converter instead of the default NodaPatternConverter<Instant> when UseNodaTime is called.

This shouldn't be a breaking change, but I've made the InstantJsonConverter public to make it easier to remove and replace with a custom converter when needed.
Let me know if you think making it internal is the better decision choice.