dsp: handle ContractNegotiationEventMessage with ACCEPTED type

[ndr-brt]

Feature Request

Currently this message is not handled by the ContractNegotiationProtocolService:

Connector/core/control-plane/control-plane-aggregate-services/src/main/java/org/eclipse/edc/connector/service/contractnegotiation/ContractNegotiationProtocolServiceImpl.java

Line 100 in 8ddcc07

throw new UnsupportedOperationException("not implemented");

it should put the ContractNegotiation in state ACCEPTED, then it should be put in AGREEING automatically, but in two different state machine iterations.

Which Areas Would Be Affected?

contract negotiation

Why Is the Feature Desired?

DSP completion