add option to stream unit updates

[rerpha]

Currently the forwarder does not allow a PV to change it's EGU field. At ISIS we need to be able to do this, but it doesn't make sense to stream the units every time a PV update occurs. We should add another schema to handle this, much like the epics alarm status and connection info schemas.

acceptance criteria

• forwarder forwards units

discussed in planning 12/06/25 at 1:17:00

[rerpha]

upstream PR: ess-dmsc#308
our fork PR: #6

[KathrynBaker]

Not waiting on the upstream PR, code is now in our fork, as all seems well.