Add XPath-style filtering ability in JSON data parser plugin

I am updating the version to 6.0.x. I also created a new MR based on that branch and the patch in #40.

I installed the MR version of migrate_plus using the instructions above on a vanilla D10 site with the minimal profile and created a test content type to use as the destination for the test migration.

I created module with a simple migration using a sample JSON file source (attached) and was successful in using a selector to limit the items which were migrated. The JSON contains an attribute named "favoriteFruit"; the item_selector in the migration selects those with favoriteFruit=strawberry. My migration only imported the items with the matching selector with no errors.

My simple migration:

id: test_data
label: Migrate Test Data

source:
  plugin: url
  data_fetcher_plugin: file
  data_parser_plugin: jsonpath
  urls:
    - modules/custom/migration_test/data/people.json

  item_selector: '$.people[?(@.favoriteFruit=strawberry)]'

  fields:
    -
      name: guid
      label: 'GUID'
      selector: guid
    -
      name: name
      label: 'Person name'
      selector: name
    -
      name: age
      label: 'Age'
      selector: age
    -
      name: gender
      label: 'Gender'
      selector: gender
    -
      name: company
      label: 'Company'
      selector: company
    -
      name: about
      label: 'About'
      selector: about
    -
      name: favorite_fruit
      label: 'Favorite Fruit'
      selector: favoriteFruit

  ids:
    guid:
      type: string

process:
  type:
    plugin: default_value
    default_value: people
  title: name
  field_age: age
  field_gender: gender
  field_company: company
  body/value: about
  body/format:
    plugin: default_value
    default_value: plain_text
  field_favorite_fruit: favorite_fruit

destination:
  plugin: 'entity:node'

migration_dependencies: { }
dependencies:
  enforced:
    module:
      - migration_test

@pixlcat:

Thanks for testing this, and for adding JSONPath links to the issue summary.

Since we looked at this issue at DrupalCon earlier this month, I am adding the tag for that.

I reviewed the changes. It mostly looks good, but I would like a little code cleanup. I am setting the status to NW, and I will re-review promptly. Let's try to get this issue done!

One of my review comments will require a bit of research and/or experimentation.

@benjifisher I guess I didn't set this back to needs review. Doing that here.