Add store methods for writing into an existing buffer #3831

Store.get takes a prototype parameter, which is used to create a buffer in which to store the fetched bytes:

I think this is not the best way to support a pattern where the caller wants to use a specific memory type. IMO it would be better if the caller was responsible to creating the buffer, and they pass a reference to that buffer to the store.

We should do this with a get_into method with the following signature:

    async def get_into(
        self,
        key: str,
        byte_range: ByteRequest | None = None,
        buf: Buffer,
    ) -> GetResult:

GetResult here is a data structure that conveys useful information about the IO operation. It would convey how much of the buffer was filled, whether the result was truncated, etc.

In concrete terms, I don't want to change the store ABC much, but I do think we could incrementally evolve our storage layer by increasingly relying on protocols. See #3758.