Make FencedFrameConfig IDL objects serializable

[blu25]

This PR:

• Makes FencedFrameConfig objects serializable and defines serializing/deserializing steps for it.
• Removes config as an attribute of a FencedFrameConfig, replacing it with a clone of the values it needs from the internal Fenced Frame Config struct.
• Passes in the sharedStorageContext directly to the navigate algorithm, which will read the sharedStorageContext from the IDL object to the embedder shared storage context in the Fenced Frame Config struct. This is done since we are no longer writing directly to the Fenced Frame Config struct in the setSharedStorageContext() algorithm.
• Modifies getNestedConfigs() to account for the changes to FencedFrameConfig.
• Update the step number for the navigate step that goes in parallel.
• Update the permissions policy introduction section to mention the internal [=fenced frame config=] struct instead of the {{FencedFrameConfig}} IDL object.

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[domfarolino]

Pretty much lgtm % one nit!