configurable SSMMessages endpoint

Would it be at all possible to use the session-manager plugin with a custom VPC endpoint for SSMMessages?

The way I understand the behavior is that the endpoint address is given as `StreamUrl` by the [SSM:StartSession](https://docs.aws.amazon.com/systems-manager/latest/APIReference/API_StartSession.html#API_StartSession_RequestSyntax) API call, which in my case is unfortunate. I am in a corporate network and DNS requests to `amazonaws.com` addresses are resolved publicly to global endpoints. I can reach our private endpoints, but only via more explicit hostnames (e.g. `vpce-123456789abcdef.ssm.eu-central-1.vpce.amazonaws.com`. I can configure aws cli to use these for STS and SSM, but not for SSM Messages. And since `SSM:StartSession` returns `wss://ssmmessages.eu-central-1.amazonaws.com` I am forced to use the global endpoint (which in my case I can't).

Would it be possible to configuration option to override the default `StreamUrl` in the session-manager-plugin? Or must it be always the `StreamUrl` returned by `SSM:StartSession` ?