Feature: Support setting connector state

### Feature description

Currently the [connector state](https://github.com/ConduitIO/conduit/blob/929fe129bbc9ea29c98b7d81b5f7eb3f39127a1a/proto/api/v1/api.proto#L86-L89) can be retrieved through the API but it can't be updated. Adding an endpoint to explicitly set the state to some specific value would allow users to change the position before starting the pipeline, giving them the power to control the starting position. This mean that an operator could externalise the storage of connector state if they want or copy over state from one Conduit instance to another.