This commit centralizes a place where the codegen excludes the Debug
trait if a shape has the sensitive trait. Previously the exclusion was
handled locally in each shape, e.g. StructureGenerator and EnumGenerator.
However, that approach may overlook a certain shape we also need to treat
as such. We now handle the exclusion of the Debug trait in one place,
BaseSymbolMetadataProvider.

This commit updates EnumGenerator and StructureGenerator based on the
change made to BaseSymbolMetadataProvider in the previous commit.
Now that the exclusion of the Debug trait was centralized, those
classes in question no longer need to do so individually.

This commit implements a custom Debug trait in BuilderGenerator now that
the derived Debug trait is excluded from BaseSymbolMetadataProvider for
the structure container. The implementation of the custom Debug trait
pretty much follows that of StructureGenerator.

This commit implements a custom Debug trait in BuilderGenerator now that
the derived Debug trait is excluded from BaseSymbolMetadataProvider for
the union container. The implementation of the custom Debug trait pretty
much follows that of EnumGenerator.

This commit implements a custom Debug trait in ServerBuilderGenerator now
that the derived Debug trait is excluded from BaseSymbolMetadataProvider
for the structure container. The implementation of the custom Debug trait
pretty much follows that of StructureGenerator.

…ndle-sensitive-trait-in-single-place

This commit updates the implementation of a custom Debug trait impl for
UnionGenerator. Turns out that in a Union, a member target can be marked
as sensitive separately outside the Union. Therefore, the implementation
of a custom Debug trait has two cases depending on where the sensitive
trait appears, either it is applied to the whole Union or to a member
target.

This commit addresses #2029 (comment).
With this change, structure shapes no longer need to exclude the Debug
trait unconditionally. The upshot is that we may be able to avoid a
custom Debug impl for a structure where the derived Debug will do, i.e.
when there is no sensitive trait either at a container level or at a
member level.

This commit addresses #2029 (comment)

This commit addresses #2029 (comment)

This commit addresses #2029 (comment)