Add logic for verifying that the STS URL is in the correct format

• At present an invalid URL can be supplied as an STS endpoint. It will attempt to create an STSClient with it and then fail with,

java.net.UnknownHostException: request session credentials: software.amazon.awssdk.core.exception.SdkClientException: Received an UnknownHostException when attempting to interact with a service. See cause for the exact endpoint that is failing to resolve. If this is happening on an endpoint that previously worked, there may be a network connectivity issue or your DNS cache could be storing endpoints for too long.:    software.amazon.awssdk.core.exception.SdkClientException: Received an UnknownHostException when attempting to interact with a service. See cause for the exact endpoint that is failing to resolve. If this is happening on an endpoint that previously worked, there may be a network connectivity issue or your DNS cache could be storing endpoints for too long.: https

* This is inefficient. An invalid URL can be parsed much earlier and can be failed based on the URL format itself.

• The error message is not very clear and does not indicate a problem in the URL format.
• In this Jira issue, we attempt to parse the STS URL and fail fast with a more relevant error message.