Add schema for engineering units update

[rerpha]

Description of Work

(as discussed in last ESS-ISIS streaming catch-up meeting)

Adds a schema for an update of engineering units for a given PV.

Units are optional in this schema as they may not exist for certain types of PVs eg. strings.

We did talk about a generic "metadata" class but it's most efficient just to use this for units.

Issue

None

Developer Checklist

• If there are new schema in this PR I have added them to the list in README.md
• If there are breaking changes to a schema, I have used a new file identifier and updated the list in README.md
• There is some documentation here or in the flat buffer file on the use case for this data, including which component is intended to send the data and/or which is the intended receiver.

Approval Criteria

This PR should not be merged until the ECDC Group Leader (acting or permanent) has given their explicit approval in the comments section.
SCIPP/DRAM should also be consulted on changes which may affect them.