add map_immutable to allow signing to work with a wrapped SdkBody (#2567)

## 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 -->
aws-sdk-rust#749

## Description
<!--- Describe your changes in detail -->
When wrapping an `SdkBody`, the resulting body is no longer signable.
Some services support this and others don't. This change adds a new
method for mapping an `SdkBody` called `map_readonly_body`. If called on
a body that is signable, it will produce a signable body. However, this
function should never when modifying the inner body is necessary, as
that will result in a signing error.

## Testing
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to -->
<!--- see how your change affects other areas of the code, etc. -->
Zelda wrote a test

## Checklist
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rather than leaving it unchecked -->
- [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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