[pigeon] add non-null type arguments

[gaaclarke]

Turns out this was only meaningful for the Dart generator. That's why most of the work is in the Dart generator.

fixes flutter/flutter#97848

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[gaaclarke]

Let me look into this a bit tomorrow. I'm think I can use a custom codec to encode Lists with non null values on the java and objc side to throw an exception if it finds a null value.

[gaaclarke]

Because of the null-safe limitation in ObjC and Java this PR has been stalled indefinitely. For example, you can't put nullability tags for types in type arguments to generic containers in ObjC or Java. I haven't found a good path forward and am not sure I recommend attempting it until we support kotlin and swift.