Allow partial updates in the API

### Feature description

Currently, our APIs mostly allow full updates only in the sense that, if certain properties are mutable, then the update request updates all of them. One example is the UpdateProcessorRequest:

```proto
message Config {
map settings = 1;
int32 workers = 2;
}

message UpdateProcessorRequest {
string id = 1;
Processor.Config config = 2;
}
```

In the processor service, the whole config is then replaced:

```go
instance.Config = cfg
```

[Link](https://github.com/ConduitIO/conduit/blob/main/pkg/processor/service.go#L175)

This implies that, if a client wants to update just the workers (for example), it first needs to first fetch the existing settings, set the new workers count, and then send the update request. We have similar examples in other APIs (pipelines, connectors).

To make the API more convenient to use, it would be good to allow partial updates that allow only certain parts of a configuration to be updated.