Issue #3600642: Add a shared GraphQlEnumValuesTrait and GraphQlBackedEnumInterface for backed GraphQL enums.

Kieran Cottrequested to merge
issue/graphql_webform-3600642:3600642-enum-values-trait into 3.x

Adds a shared GraphQlEnumValuesTrait and GraphQlBackedEnumInterface for GraphQL-backed PHP enums.

Several enum classes were carrying identical toArray() implementations to expose GraphQL enum values as associative arrays. This change centralises that logic in one trait, keeps the existing toGraphQlEnumValue() mappings on each enum, and updates the eight affected enums to use the shared implementation.

The schema-facing behaviour is unchanged: enum names continue to use the existing GraphQL values, and backing values are still returned in the same shape as before.

Closes #3600642

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