Create ProxyBuffer RPC methods to check if payloads have been processed by the registry

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ProxyBuffer exposes two methods for registering devices: `RegisterDevice` and `BatchRegisterDevice`. Internally, these register the device(s) in an internal SQLite database. Additionally, the service has an async process (syncer) that periodically reads the local database to find unregistered records, send them to an external registry, and mark them as registered.

However, we don't have a way of figuring out which records have already been processed in the registry besides manually querying the internal database (which might not be an easy thing to do depending on the SQLite config).

We should have additional RPC methods in ProxyBuffer to check this status: something like `CheckDeviceStatus` and `BatchCheckDeviceStatus`.

Start by reading ProxyBuffer's RegisterDevice and BatchRegisterDevice methods, then trace the syncer and its internal SQLite records to understand how registry processing is recorded. Define the individual and batch status responses and add RPCs that report whether each payload has been processed by the external registry.

Written by the indexing model from the issue text.