8210 importddi fix

[lubitchv]

What this PR does / why we need it:
importddi is broken because ddi xml does not have corresponding subject field. The fix is by filling Subject field with "N/A" similar to Sword Api.

Which issue(s) this PR closes:

• closes Native API: Import ddi dataset does not appear to be working, fails on parsing subject. #8210
• closes Native API: Import ddi dataset example needs a sample ddi file like the json example has. #8209
• closes DDI XML cannot be imported now that collectionMode has been made multiple (in PR #8473) #8489

Suggestions on how to test this:
testImportDDI was added to DataversesIT.
One can also test by using curl:
curl -H X-Dataverse-key:$API_TOKEN -X POST $SERVER_URL/api/dataverses/$DATAVERSE_ID/datasets/:importddi --upload-file ddi_dataset.xml
Sample xml file is in doc/sphinx-guides/source/_static/api/ddi_dataset.xml

Does this PR introduce a user interface change? If mockups are available, please link/include them here:
No

Is there a release notes update needed for this change?:
Release notes update is added to doc/release-notes/8210-importddi-fix.md

Additional documentation:
Documentation added to Native Api guide

[coveralls]

Coverage increased (+0.0003%) to 18.864% when pulling f23809d on lubitchv:8210-importddi-fix into dbca4ab on IQSS:develop.

[pdurbin]

Overall, this looks great.

I did suggest a doc change, so please take a look.

The test is failing but it's our fault. Please hold on until we resolve #8489.

[pdurbin]

@lubitchv @landreev @qqmyers @scolapasta we've been discussing in IQSS and community Slack but not all at once. Let me try to get us all on the same page.

@landreev made pull request #8491 to address the collectionMode change that affects this pull request about DDI import. As he notes in the description, it's hard to test it since DDI import is broken. So, I went ahead and merged it into this pull request. The new testImportDDI test runs fine on my laptop. I'll wait until Jenkins is happy with it before moving this pull request to QA. Obviously, if anyone else wants to do further review of this pull request, please go ahead.

Meanwhile, @qqmyers also has a change for scripts/api/data/dataset-create-new-all-default-fields.json in pull request #8493. I think we should merge this pull request (#8483) first and then merge develop into his.

As for this pull request, I think I'm done making changes. Thank you @lubitchv for committing my doc suggestion. That last thing I haven't mentioned is that I removed one of the test XML files since they're identical.

[pdurbin]

The new "import DDI" test just passed on Jenkins. Moving to QA.