Replace `enforce_order(bool)` with `enum RuleMode` (#3502)

RuleMode describes how rules will be interpreted.
- In RuleMode::MatchAny, the first matching rule will be applied, and
the rules will remain unchanged.
- In RuleMode::Sequential, the first matching rule will be applied, and
that rule will be removed from the list of rules.

Also adds a `make_client!` macro produces a Client configured with a
number of Rules and appropriate test default configuration.

## Motivation and Context
Working through improvements on experimental mocks after implementing
them in the Cloudwatch Logs example.

## Testing
Unit tests, doctests, 

## Checklist
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [ ] 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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