Optionally use the new keyset implementation [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

This MR adds a feature flag (new_graphql_keyset_pagination) to use the new keyset pagination backend for GraphQL when it's possible.

The MR also adds a builder class (SimpleOrderBuilder) that transforms simple order_values into a keyset (Gitlab::Pagination::Keyset::Order) compatible format, without rewriting all order() scopes in the models.

In some cases (raw SQL order, nulls last, function based ordering) automatic conversion is not possible, in this case we fall back to the existing pagination (Gitlab::Graphql::Pagination::Keyset::Connection) code.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because the change is behind a feature flag.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team