allow templating iframe URL with current token

[btkostner]

Right now the ha-panel-iframe component accepts a simple URL that gets passed to the iframe. What I'm trying to accomplish is have an iframe that points to an external system, where I'd like to bypass authentication by using the already existing home assistant token from the frontend.

My idea is to have a bool prop called templated_url that when true, will use lodash.template on the url with the current token as a variable. This would let you do something like https://example.com?ha_token={{ current_token }} in the url field.

If this sounds like an acceptable idea, I'd love to open some PRs trying to add it.