feat: support signed embed urls during backend session creation

[daniel-lopes-optimizely]

This would unblock our team's use case, where we want to exclusively initiate a session for a particular LookML component (e.g. dashboard) and because there's sensitive information we don't want to pass to the frontend to generate the url , we need to generate the embed signed url in the backend and pass it back to the frontend running Embed SDK.

Let me know if this is PR is enough or if we need any additional work done; this is quite important for us.

We're in contact with the Looker Engineering team, so they can also provide information over our several use cases that are out of norm and may require additional context.

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