Add param option to exclude basic metadata from GET content

[razvanMiu]

This PR will add two more optional parameters to GET content:

• include_basic_metadata - by default this is True; If False all metadatas except @id, @type and is_folderish will be excluded from result allowing metadata_fields parameter to specify the desired metadatas
• include_expandable_elements - by default this is True; If False the expandable elements will not be included in the result

Ref #1661

[mister-roboto]

@razvanMiu thanks for creating this Pull Request and helping to improve Plone!

TL;DR: Finish pushing changes, pass all other checks, then paste a comment:

@jenkins-plone-org please run jobs

To ensure that these changes do not break other parts of Plone, the Plone test suite matrix needs to pass, but it takes 30-60 min. Other CI checks are usually much faster and the Plone Jenkins resources are limited, so when done pushing changes and all other checks pass either start all Jenkins PR jobs yourself, or simply add the comment above in this PR to start all the jobs automatically.

Happy hacking!

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[davisagli]

@tisto There is some discussion in the linked issue: https://github.com/plone/plone.restapi/issues/1661