Rename `make_token` to `idempotency_token_provider` (#2783)

## Motivation and Context
Incorporates suggestion made in
https://github.com/awslabs/smithy-rs/pull/2762#discussion_r1231462878



## Description
This PR renames `make_token` to `idempotency_token_provider` for clarity
wherever it is referred to in the fields and API in service configs and
their builders.

## Testing
Existing tests in CI

## 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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Co-authored-by: Yuki Saito <awsaito@amazon.com>