GraphQL: Subclass offset based pagination

What does this MR do?

Keyset / stable cursor based pagination currently has limits, and can not be used in all ActiveRecord::Relation cases. However by default we use keyset pagination for every ActiveRecord::Relation.

A new Gitlab::Graphql::Pagination::OffsetActiveRecordRelationConnection simply subclasses from the graphql gem GraphQL::Relay::RelationConnection, and allows us to use offset pagination when necessary. Many times it is good enough, and allows us to support reasonable pagination in cases that KeySet pagination doesn't support.

Once we upgrade to 1.10.x in #207278 (closed), we'l migrate the class to use the new GraphQL::Pagination::ActiveRecordRelationConnection.

The reason for putting this class in gitlab/graphql/pagination instead of gitlab/graphql/connections is because 1.10.x now places connections in the pagination directory. An upcoming MR will refactor other connections into this directory structure.

Related to #36758 (closed)

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Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
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