Add NoNewWork node command to tell free nodes to stop asking for new work

[bmc-msft]

Currently, there are two states that are non-obvious as to what happens next.

1. A free node is "out of date". Right now, it will stay on the old version until it tries to launch a task, at which point the node will check "can I run this work". When the node asks "can I run this work", the node is rejected and then moves into the reimage/delete state.
2. A free node is part of a shrinking scaleset. Right now, it will wait until a task launch similar to the "out of date" node to start the shrinking.

This PR adds sending "no_new_work" command to Nodes which basically allows us to tell free nodes to stop.

NOTE: The node command infra in the supervisor will fail badly if it receives an unsupported command. As such, this includes "minimum supported version" support that gate sending the message such that we don't accidentally break nodes that are actively doing work.

[bmc-msft]

Note, as is this currently checks is the minimum version 2.16.1, which is the next possible release this could go into.

If we make a 2.16.1 that doesn't include this value, we should bump it.

[bmc-msft]

Manual testing done:

• Verified that the no_new_work messages don't get sent to older nodes.
• Verified that nodes already running work do not stop because of these messages.
• Verified that nodes not running work stop as expected.