ECS Server support auto-fetch of IAM creds

[synfinatic]

ECS Server should support a CLI option which will cause it to automatically pull/refresh IAM credentials without requiring manual loading. Would mean that starting the ECS server would trigger the SSO auth workflow if necessary.

This is a challenging request because:

1. Currently the ecs server doesn't load the config, which is necessary to map profile/role to an SSO provider
2. Users may have multiple SSO providers
3. ecs server has no provisions for reading/writing to the SecureStore

Blocked by: https://github.com/synfinatic/aws-sso-cli/issues/936

[synfinatic]

Dropped priority since AWS bug prevents SSL support which makes the ECS server less interesting at the moment.