Remove ProviderConfig as the configuration source for AssumeRoleProvider (#3014)

## Motivation and Context
`AssumeRoleProvider` currently uses `ProviderConfig` as a source of
configuration, but that API is hard use and not intended for external
consumption.

This fixes the Assume Role issue but only for `AssumeRoleProvider`

## Description
Update the API (see changelog) to be more ergonomic and derive
configuration from `SdkConfig` instead.

## Testing
Existing tests + new unit tests

## 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._