Allow user to set `policy` and `policy_arns` in...

Allow user to set `policy` and `policy_arns` in `WebIdentityTokenCredentialsProvider` builder (#3506)

Related PR: https://github.com/smithy-lang/smithy-rs/pull/1892/



## Motivation and Context
This change allows users to define inline IAM policies and/or set
predefined policies (using their ARNs) with
`WebIdentityTokenCredentialsProvider`

## Description
Adds `policy` and `policy_arns` to `WebIdentityTokenCredentialsProvider`
builder.

## Testing

## 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: ysaito1001 <awsaito@amazon.com>