Add Support for Temporary Credentials in Grafana Cloud

[sarahzinger]

Adding support for experimental/beta feature to allow the use of AWS Temporary Credentials in Grafana Cloud.

To test locally :

• in main grafana enable the feature toggle awsDatasourcesTempCredentials
• npm link to the latest version of main of https://github.com/grafana/grafana-aws-sdk-react (assuming it hasn't been published yet)
• pull these changes locally in your version of grafana-aws-sdk
• add a line to go.mod in main grafana to connect to your local version of the sdk like so replace github.com/grafana/grafana-aws-sd => /Users/yourfilepathtothegrafana-aws-sdk
• add a long term aws user's access key and secret key to a .aws/credentials file under the profile name [assume_role_credentials] (sarah or ida can point you to an existing one if you need)
• make sure that user has sts permissions
• create a role (not user!) in aws with the required cloudwatch permissions. Create a trust relationship with the AWS account that has the long term credentials. Choose an externalId. (Alternatively if you want to use one Sarah and Ida have been using we can point you to it)
• Set that externalId in the custom.ini

You should now be able to auth with the new GrafanaAssumeRole feature.

[idastambuk]

Tested it in local and it works with the creds file and externalId. Let's see tomorrow what we can do about the backend feature toggle check though!