Update feature for binary chunked records

[labheshd]

Is your feature request related to a problem? Please describe.
Currently, open saves supports creating a new binary chunk record and retrieval of the binary record. There is no option to UPDATE an existing binary record with new chunks.

Describe the solution you'd like
The feature request on open saves is to provide a new endpoint for allowing update operations on existing chunked binary records using the same key. This way a game client can update the chunks for a given a record, basically a patch operation.

Describe alternatives you've considered
An alternative we are currently using is to delete an existing record, and then re-upload using the same key. But of course this is not ideal and will run into many issues, ex: what if the client loses connection after delete, or before the re-upload happens.

Additional context
Example use case: A game, on its first boot, uploads a save game file using a record key: SaveGame. On the second game boot, the player made more progress in the game and wants to upload the new progress to the save game file.
Currently, open saves will error out saying a conflicting record already exists.
Ideally, the game uses a PATCH operation to update the SaveGame record key, with new binary chunks that will note the new progress of the player.