Make allow attribute configurable in iframe panel

[tomayac]

I have a webapp that I run in the iframe widget that needs WebUSB access, but unfortunately the allow attribute is hardcoded to just "fullscreen":

frontend/src/panels/iframe/ha-panel-iframe.ts

Line 44 in 1dc6132

allow="fullscreen"

Would it be possible to make the allow attribute configurable, or to change the value to "fullscreen; usb" as per the iframe syntax? I'd probably prefer the former, since it's more flexible and also more secure than an additional possibly powerful feature.

[tomayac]

See #19087 for a suggested change.