Add store methods for writing into an existing buffer

[d-v-b]

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

zarr-python/src/zarr/abc/store.py

Lines 196 to 200 in 690c5bb

	async def get(
	self,
	key: str,
	prototype: BufferPrototype,
	byte_range: ByteRequest | None = None,

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.

[d-v-b]

codecs should probably also have encode_into and decode_into, that follow the same pattern.

[ilan-gold]

codecs should probably also have encode_into and decode_into, that follow the same pattern.

This is directly relevant for zarrs style unordered-via-threading performance/behavior i.e., the ShardingCodec can encode directly to an output buffer without caring about its order