Fenced frames APIs specific to shared storage should be specified in this spec

[domfarolino]

Fenced frames recently got feedback from the TAG that shared storage seemed like an explicit dependency of the fenced frame specification due to some of the shared-storage-specific APIs appearing in that specification:

• https://wicg.github.io/fenced-frame/#fenced-frame-config-embedder-shared-storage-context
• https://wicg.github.io/fenced-frame/#dom-fencedframeconfig-setsharedstoragecontext

If fenced frames has the opportunity to ascend to something like the WHATWG before this specification, for example, then the fenced frame feature should probably not normatively include APIs that are specific to this feature. Therefore I think it's best if the bits above were specified in the shared storage spec as extensions of the fenced frame specification.