Extract builderInstantiator interface to prepare for nullability changes (#2988)

## Motivation and Context
#1725 exposed the need for easily configuring builder behavior between
client & server

## Description
- extract builderGenerator interface and attach it to `codegenContext`
to avoid loads of threading it up and down

## Testing
- codegen unit tests
- [x] codegen diff audit

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

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