Block forging state query functionality

[tdammers]

#1402 introduces KES agent connectivity.

With this, a node process can be in some states that weren't possible previously, to wit:

• "Not a block forging node": the node isn't configured to forge any blocks, and thus doesn't do so.
• "Block forging, connected": this is the normal block forging state - the node is connected to a KES agent, holds a valid KES key, and participates in block forging.
• "Block forging, not connected": the node is not currently connected to a KES agent, but holds a valid KES key, and participates in block forging using that key.
• "No KES key": the node is connected to a KES agent, but hasn't received a KES key yet, so it cannot forge any blocks. Once the connected KES agent pushes a key, the node will start forging blocks.
• "Not connected": the node is not connected to a KES agent, and doesn't hold a valid KES key, so it cannot forge any blocks.

These states will also be evident from trace logs, but for debugging and active monitoring purposes, it may be useful to allow users to query the node for its block forging state.