codegen: SDK use API model to render parameters as value instead of pointers types

[jasdel]

The Smithy API model provides information for the SDK's code generator to determine if a API input/output parameter must be nullable (aka optional), and when the parameter isn't nullable (aka unboxed). With this information the SDK's code generator can opportunistically render some parameters as value types instead of rendering all parameters as pointer types.

This information directly impacts how the SDK renders the following types.

• List and Map members can be rendered as value if not decorated with @sparse trait
  • []string instead of []*string
  • map[string]string instead of map[string]*string
• Number types (float32, int32, etc) struct members can be values.
• Boolean struct members can be values.

In addition to this change the SDK should ensure it doesn't serialize number and bool parameters of structs that are unboxed.

Related issues

• Introduce the concept of a NullableIndex and Pointable Index codegen: Introduce the concept of a NullableIndex and Pointable Index smithy-go#232
• Lists members should be generated as values by default Lists members should be generated as values by default smithy-go#128
• Boolean and Number shapes can be decorated as boxed or unboxed Boolean and Number shapes can be decorated as boxed or unboxed smithy-go#127

[github-actions]

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