ClientAuthenticationFactory should enable to set the region for AWS IAM Authentication

[Amuerte]

In some cases, the Vault instance which is making the proxy call to AWS, is configured to use the global STS endpoint (on us-east-1), which is the default behavior. But when your application is deployed in another region, the authentication will fail as spring-vault use the region from your application and the request to AWS are signed with that region.

As a consequence, you end up with an application deployed in one region and a Vault instance using the global STS endpoint, without no easy way to use the spring-vault client to login.

I propose to enable the region configuration in the ClientAuthenticationFactory and will be glad to help with a PR.

PS:
The problem was initially discussed in this issue from spring-vault.