Add units to number visualization

[maxiadlovskii]

Description

This PR adds units to a single number widget as we have on other idgets

Motivation and Context

fix: #20965

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

[dennisoelkers]

When I remove the unit of a metric and try to save the dashboard, I get this error from the backend:

{
    "type": "RequestError",
    "message": "Error at \"state.d90f73b7-a11b-46c3-b724-5a15204caadd.widgets.[0].units.took_ms.unit_type\" [1, 520]: Must be of type String",
    "line": 1,
    "column": 520,
    "path": "state.d90f73b7-a11b-46c3-b724-5a15204caadd.widgets.[0].units.took_ms.unit_type",
    "reference_path": "org.graylog.plugins.views.search.views.$AutoValue_ViewDTO$Builder[\"state\"]->java.util.LinkedHashMap[\"d90f73b7-a11b-46c3-b724-5a15204caadd\"]->org.graylog.plugins.views.search.views.$AutoValue_ViewStateDTO$Builder[\"widgets\"]->java.util.HashSet[0]->org.graylog.plugins.views.search.views.widgets.aggregation.AutoValue_AggregationConfigDTO$Builder[\"units\"]->java.util.LinkedHashMap[\"took_ms\"]->org.graylog.plugins.formatting.units.model.UnitId[\"unit_type\"]"
}

This is how the units attribute looks like in the view state:

{
    "took_ms": {}
}

[dennisoelkers]

Looks good, works fine. Issue is fixed in separate PR.