add `AWS_PROFILE` support by allowing async configuration finalization

In the following PR AWS_PROFILE support was dropped, as it did not fit into the scope of the changes:

• https://github.com/maplibre/martin/pull/2251

Reintroducing AWS_PROFILE will require a smaller refactoring, changing the function signature from:

fn finish(&mut self) -> ConfigurationResult<()>

to

async fn finish(&mut self) -> ConfigurationResult<()>

This change will have follow-on effects throughout the codebase.

Tasks:

• Refactor finish to be async and propagate changes.
• add AWS_PROFILE support (and respective profile/aws_profile config variable) via the aws-config crate
• Validate integration and update relevant documentation.