State Proof Incremental Load REST API

[winder]

State Proof keys are large enough that we need a way to load them incrementally by round.

The main parts here are:

1. New REST endpoint:
   POST /v2/participation/{id}

The body is not fully defined, but it may not need to be anything more than an array:

[{
    "round": 10,
    "key": <data>
},
{
    "round": 11,
    "key": <data>
}]

Related
Update the goal cmds (#2594)

Acceptance criteria

• Update the key via REST API (this story will cover this task)