[chore] refactor extractFromCtx a bit

[tsmethurst]

A little chore PR that comes out of the Move stuff I've been doing. PRing it separately here to avoid the Move PR becoming too big.

Just refactors extractFromCtx -> getActivityContext and simplifies the return type. No actual logic changes, but there might be in the Move PR.

[daenney]

This looks good to me, it's just moving the data we got returned by the function into a struct.

One thing maybe for the future, if you change the requestingAcct and receivingAcct variables that are created in function bodies to be requestingAccount and receivingAccount to retain the same names as we originally had, the diff becomes a lot smaller making this kind of change a bit simpler to review.

[tsmethurst]

Ah yes, I actually did that in a previous version of this, but I ended up preferring the brevity of requestingAcct compared to requestingAccount etc. Bit easier on the eyes and we don't use acct to mean anything else.