Budget metadata should be consistent between Shared Storage + Fenced Frame specs

[blu25]

Currently the fenced frame config has the notion of an exfiltration budget metadata, which is copied directly from the implementation's definition. There is an open PR to rename that to "shared storage selectURL budget metadata" (WICG/fenced-frame#138), but it is currently not being referenced by anything in the Shared Storage spec.

The Shared Storage spec does have a notion of a shared storage page budget and an open issue to add a "pending shared storage budget debit" to the fenced frame config (#148).

Which ever permutation of these we end up ultimately using should be consistent across the 2 specs.