We require configuration to preform updates (as well as previews,
destroys and refreshes). Because of how everything evolved, loading
this configuration (and finding the coresponding decrypter) was
implemented in both the file and http backends, which wasn't great.
Refactor things such that the CLI itself builds out this information
and passes it along to the backend to preform operations. This means
less code duplicated between backends and less places the backend
assume things about the existence of `Pulumi.yaml` files and in
general makes the interface more plesent to use for others uses.
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