fix: use nullable types for all structure members (Java, .NET)

[alex-chew]

Issue #, if available: #278

Description of changes: Changes the .NET and Java code generators to use nullable types for all structure members (even required members with primitive-equivalent shapes).

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[alex-chew]

Marking this as ready for review despite the unrelated failing CI test (something to do with a Rust-related patch not applying, which we believe is due to unintended nondeterminism in the module order emitted by the Dafny-to-Rust compiler).

[robin-aws]

Leaving an initial blocking review just because I'm not worried that this isn't correct for Smithy 1.0 models, that the better fix is to support @box consistently (it is supported for Java but not .NET AFAICT)

[texastony]

An issue with going to down further down this path is that Smithy-Dafny is forking the Smithy 1.0 Specification.

Smithy-Dafny could argue that such a change in behavior is necessary to support the Local Service Trait;
but I am not certain that argument is valid,
as I think the Local Service Trait can work without this forking of behavior.

A stronger argument, in favor of the box approach,
or maybe just in dropping Smithy 1.0 and upgrading to 2.0,
is that all Smithy Code Generators should behave same by following the Smithy Specification.