codegen: create RPC service interface with gRPC & DRPC adapters

[shubhamdhama]

stacked on #2
Define a shared RPC<Service>Client interface to wrap both gRPC's
<Service>Client and DRPC's DRPC<Service>Client. Streaming methods will
also return the common RPC<Service>_<Method>Client.

Adds two simple adapters:

• NewGRPC<Service>ClientAdapter
• NewDRPC<Service>ClientAdapter

They implement RPC<Service>Client, so we can swap clients at runtime via
the cluster setting.

[shubhamdhama]

Example code generated by this:

// MultiRaft gRPC -> RPC adapter
type grpcMultiRaftClientAdapter multiRaftClient

func NewGRPCMultiRaftClientAdapter(conn *grpc.ClientConn) RPCMultiRaftClient {
	return (*grpcMultiRaftClientAdapter)(&multiRaftClient{conn})
}

func (a *grpcMultiRaftClientAdapter) RaftMessageBatch(ctx context.Context) (RPCMultiRaft_RaftMessageBatchClient, error) {
	return (*multiRaftClient)(a).RaftMessageBatch(ctx)
}

func (a *grpcMultiRaftClientAdapter) RaftSnapshot(ctx context.Context) (RPCMultiRaft_RaftSnapshotClient, error) {
	return (*multiRaftClient)(a).RaftSnapshot(ctx)
}

func (a *grpcMultiRaftClientAdapter) DelegateRaftSnapshot(ctx context.Context) (RPCMultiRaft_DelegateRaftSnapshotClient, error) {
	return (*multiRaftClient)(a).DelegateRaftSnapshot(ctx)
}

// compile-time assertion
var _ RPCMultiRaftClient = (*grpcMultiRaftClientAdapter)(nil)

// MultiRaft DRPC -> RPC adapter
type drpcMultiRaftClientAdapter drpcMultiRaftClient

func NewDRPCMultiRaftClientAdapter(conn drpc.Conn) RPCMultiRaftClient {
	return (*drpcMultiRaftClientAdapter)(&drpcMultiRaftClient{conn})
}

func (a *drpcMultiRaftClientAdapter) RaftMessageBatch(ctx context.Context) (RPCMultiRaft_RaftMessageBatchClient, error) {
	return (*drpcMultiRaftClient)(a).RaftMessageBatch(ctx)
}

func (a *drpcMultiRaftClientAdapter) RaftSnapshot(ctx context.Context) (RPCMultiRaft_RaftSnapshotClient, error) {
	return (*drpcMultiRaftClient)(a).RaftSnapshot(ctx)
}

func (a *drpcMultiRaftClientAdapter) DelegateRaftSnapshot(ctx context.Context) (RPCMultiRaft_DelegateRaftSnapshotClient, error) {
	return (*drpcMultiRaftClient)(a).DelegateRaftSnapshot(ctx)
}

// compile-time assertion
var _ RPCMultiRaftClient = (*drpcMultiRaftClientAdapter)(nil)

[shubhamdhama]

Same as #2 (comment) here for why tests are missing for these changes.

[cthumuluru-crdb]

LGTM!