Make required context parameters required (#2964)

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
<!--- If it fixes an open issue, please link to the issue here -->
When a `@contextParam` is marked as required, we will enforce it on
inputs. Since these fields may influence endpoint, omitting them can
result in a different target being hit.

- #1668 
- aws-sdk-rust#873

## Description
<!--- Describe your changes in detail -->

## Testing
- [x] S3 Integration test

## 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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