hwp-supervisor: Add missing shutdown procedure

[bbixler500]

This PR adds functionality to the hwp-supervisor agent to shut off bias power to the driver board during a triggered shutdown

Description

Currently, when a HWP shutdown is triggered the only actions taken is to lock out user controls and shut off the drive power. These actions alone are not enough to fully stop the HWP as the driver board bias power needs to be disabled as well (we should be doing this anyways because the electronics shouldn't be biased if there is expected to be large swings in the cryostat temperature). This PR adds to the hwp-supervisor's monitor process additional logic which shuts off the bias power once a shutdown is triggered.

Motivation and Context

See above

How Has This Been Tested?

This was tested successfully on satp2 by simulating a communication failure to initiate the shutdown

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[BrianJKoopman]

This looks good, though I have one suggestion on the implementation that should avoid cancel_shutdown being called when it doesn't really apply.

[BrianJKoopman]

Alright, thanks for answering those questions. Looks good to me!