Make service config just contain a `FrozenLayer` in the orchestrator mode (#2762)

## Motivation and Context
Service config structs now only contain a
`aws_smithy_types::config_bag::FrozenLayer` in the orchestrator mode (no
changes for the middleware mode).

## Description
This PR reduces the individual fields of service configs to contain just
`FrozenLayer`. This makes service configs work more seamlessly with
runtime plugins in the orchestrator mode.

Note that service config _builder_ s still contain individual fields.
We're planning to make them just contain
`aws_smithy_types::config_bag::Layer`. To do that, though, we need to
make `Layer` cloneable and that will be handled in a separate PR. For
builders, the only change you will in the PR is that their `build`
method will put fields into a `Layer`, freeze it, and pass it to service
configs.

This PR is marked as a breaking change because it's based on [another
PR](https://github.com/awslabs/smithy-rs/pull/2728

) that's also breaking
change.

## Testing
- [x] Passed tests in CI

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Co-authored-by: Yuki Saito <awsaito@amazon.com>
Co-authored-by: Zelda Hessler <zhessler@amazon.com>