Write API: make TupleKeys more canonical

Today the Write API looks like:

message WriteRequest {
    string store_id = 1;
    TupleKeys writes = 2;
    TupleKeys deletes = 3;
    string authorization_model_id = 4;
}

message TupleKeys {
    repeated TupleKey tuple_keys = 1;
}

message TupleKey {
    string object = 1;
    string relation = 2;
    string user = 3;
}

The usage of TupleKeys is completely unnecessary and makes the API and related code stutter. For example in Go code we have:

tupleKeys := openfgapb.TupleKeys{
    TupleKeys: []*openfgapb.TupleKey{...}
}

I propose we make this more canonical with simply:

message WriteRequest {
    string store_id = 1;
    repeated TupleKey writes = 2;
    repeated TupleKey deletes = 3;
    string authorization_model_id = 4;
}

and the corresponding code in Go will simply look like:

tupleKeys := []*openfgapb.TupleKey{...}

Note that these changes are a breaking change to the API though. The HTTP request body and gRPC request bodies change in an incompatible way.

{
    "writes": {
        "tuple_keys": [...]
    },
    "deletes": {
        "tuple_keys": [...]
    }
}

becomes

{
    "writes": [...],
    "deletes": [...]
}

Start by locating the protobuf definitions for WriteRequest, TupleKeys, and TupleKey, then inspect the Go code generated from them and the HTTP and gRPC request handling. The work is done when writes and deletes use repeated TupleKey values directly and both request formats consistently use the proposed shapes, with affected callers and tests updated.