Feature/104 unit query api

Defines an optional core schema for measurement.filtering which extends Query.units with a filter parameter.

The intention with the design of this input type is to require minimal bindings to the OM2 ontology in order to facilitate simpler implementation and reduce integration complexity. But, I would like a review on the chosen parameters to ensure we don't feel as though any flexibility or additional possibilities are being forfeited.

lib/schemas/bridging/measurement.filtering.gql

+##
+#
+# Query API parameters for Unit records
+#
+# Allows inference and retrieval on the basis of properties defined in the OM2
+# (Ontology of units of Measure 2.0)
+# @see http://www.ontology-of-units-of-measure.org/resource/om-2/
+# @see https://github.com/HajoRijgersberg/OM
+#
+# @package  Valueflows GraphQL
+# @since    2023-04-04
+#
+##
+
+input UnitFilterParams {
+  "Search for any matching `Unit` `label`s"
+  searchString: String
+
+  "Match `Unit`s by standard symbol (eg. `m`, `ft`, `gal`). For custom and standard `Unit`s the `symbol` must match exactly as provided."
+  symbol: [String!]
+
+  """
+  Query `Unit`s matching well-defined semantic web ontology terms.
+
+  For well-known `Unit`s, these URIs are usually prefixed with `om:` referencing
+  OM2 vocabulary terms (for example, `om:metre`, `om:gallon-US`).
+
+  @see http://www.ontology-of-units-of-measure.org/resource/om-2/
+  @see https://github.com/HajoRijgersberg/OM
+  """
+  sameAs: [URI!]
+
+  """
+  Filter `Unit`s to those which are instances of the given `rdfs:Class` URI.
+
+  In systems which expose this metadata, this can be useful for fetching only
+  standard `Unit`s (defined in OM2) by matching against `instanceOf: ['om:Unit']`.
+
+  Similarly, bulk retrieval of domain-specific `Unit` records is provided
+  (i.e. `instanceOf: ['myApp:Unit']`), as well as loading sub-domains of the OM
+  model (i.e. `instanceOf: ['om:VolumeUnit']`).
+
+  @see http://www.ontology-of-units-of-measure.org/resource/om-2/Unit
+  """
+  instanceOf: [URI!]
+}
+
+type Query {
+  units(filter: UnitFilterParams): UnitConnection!
+}