codegen: Add support for list and maps generated as value members instead of pointer

[jasdel]

Updates the SDK code generation to make use of the new smithy NullableIndex and smithy-go's GoPointableIndex. These utilities allow the SDK's code generation to be aware of API shapes and members that are nullable, and can be rendered as values by the code generation instead of pointers.

This update updates the SDK's code generation types to be value instead of pointer in most cases.

• Numbers (int8, int16, int32, int64, float32, float64)
• bool
• lists - e.g. []string instead of []*string
• maps - e.g. map[string]string instead of map[string]*string.

Number and boolean types that are not boxed (not pointer) will not serialize their zero value unless they are members of a list or map.

This change also significantly refactors how the SDK represents nullability and pointability when generation types and members from an API model. Utilities were added in the smithy-go change that help ensure more consistent logic through the sdk's code generation.

Depends on aws/smithy-go#232
Fixes #896

[skmcgrail]

Have you ran make integration to verify these changes?

[jasdel]

Yeah verified that all integration tests pass successfully.

[skotambkar]

Looks like smithy-go dep version is not correct? CI for Java fails as it doesn't find methods from aws/smithy-go#232

[jasdel]

That's probably result of bad merge. There where no Go changes in the smithy-go change. I won't be able to have CI succeed on this PR until the smithy change is merged because it depends on Java changes in that pr.