fix(ra-data-graphql-simple): Unify meta location in buildVariables

[dricholm]

Problem

Fixes #10317.
The sparse fields functionality works by reading the meta.sparseFields array. But for create and update mutations it looked for the sparseFields value in data.meta.

Solution

Return the meta object in buildCreateUpdateVariables which is used for both create and update mutations.

How To Test

Can be tested by setting the meta object for a create or update mutation and setting sparse fields on it:

{
  meta: { sparseFields: ['id'] }
}

Trigger the mutation and the generated selection should only include fields specified in sparseFields.

Additional Checks

• The PR targets master for a bugfix, or next for a feature
• The PR includes unit tests (if not possible, describe why)
• The PR includes one or several stories (if not possible, describe why)
  • Not UI related, affects data provider. I'm not aware that it has any stories.
• The documentation is up to date

[djhi]

Thanks!