Add get_foo() -> &Option<Foo> accessors for builders and fluent builders. (#2792)

This allows testing, as well as making decisions while creating
builders. The references are not mutable, so these props remain read
only keeping the Builder invariants.

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
Some operations are built up using various logic (for instance, choosing
an S3 bucket based on a logged in user). It is a lot of overhead to test
at the mock level, and the Debug formatting is typically not stable.
<!--- If it fixes an open issue, please link to the issue here -->
Closes #2791 

## Description
<!--- Describe your changes in detail -->
1. Add `get_foo(&self) -> &Option<Foo>` to Builders
2. Add `as_input(&self) -> &OperationInputBuilder` to Fluent Builders
3. Add `get_foo(&self) -> &Option<Foo>` to FluentBuilder which delegates
to Builders


## Testing
<!--- Please describe in detail how you tested your changes -->
Included unit tests, as well as [proof of concept
tests](https://github.com/DavidSouther/aws-doc-sdk-examples/commit/017e8a2e407eba6a4c259bca0334f1356ab822d9

)
<!--- Include details of your testing environment, and the tests you ran
to -->
<!--- see how your change affects other areas of the code, etc. -->

## 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

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._

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Co-authored-by: John DiSanti <jdisanti@amazon.com>