Fix Rust 1.83.0 "elided_named_lifetimes" warning. (#3955)

Compiling aws-config v1.5.12 (.../aws-config)
warning: elided lifetime has a name
   --> .../aws-config/src/meta/credentials/chain.rs:117:72
    |
117 | fn provide_credentials<'a>(&'a self) ->
future::ProvideCredentials<'_>
| -- lifetime `'a` declared here ^^ this elided lifetime gets resolved
as `'a`
    |
    = note: `#[warn(elided_named_lifetimes)]` on by default

warning: elided lifetime has a name
   --> .../aws-config/src/meta/token.rs:108:60
    |
108 |     fn provide_token<'a>(&'a self) -> future::ProvideToken<'_>
| -- lifetime `'a` declared here ^^ this elided lifetime gets resolved
as `'a`

warning: elided lifetime has a name
   --> .../aws-config/src/sts/assume_role.rs:317:72
    |
317 | fn provide_credentials<'a>(&'a self) ->
future::ProvideCredentials<'_>
| -- lifetime `'a` declared here ^^ this elided lifetime gets resolved
as `'a`

## Motivation and Context
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## Description
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## Testing
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## Checklist
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- [ ] For changes to the smithy-rs codegen or runtime crates, I have
created a changelog entry Markdown file in the `.changelog` directory,
specifying "client," "server," or both in the `applies_to` key.
- [ ] For changes to the AWS SDK, generated SDK code, or SDK runtime
crates, I have created a changelog entry Markdown file in the
`.changelog` directory, specifying "aws-sdk-rust" in the `applies_to`
key.

----

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---------

Co-authored-by: Landon James <lnj@amazon.com>