Add API to retrieve embedder context from inside a fenced frame

[pythagoraskitty]

We propose adding to the API surfaces of both Shared Storage and Fenced Frames so that a string of contextual information can be written by the fenced frames's embedder and then retrieved inside a shared storage worklet nested in the fenced frame.

[pythagoraskitty]

jkarlin@ and xyaoinum@, could you please take a look? Thank you.