Add conversion of dict to string during Qtable construction

[stscieisenhamer]

WIP RCAL-798

This PR addresses a bug during Qtable construction for the MosiacImage individual_image_meta block.

While creating the Qtable for the individual_image_meta block, if an input meta has deeply nested nodes, these nodes, appropriately, are turned into dicts. However, the Qtable column for these situations becomes just a generic object, causing validation errors.

Taking the cue of the conversion of lists to strings, the same approach is taken for dicts. Checklist

• [x] Added entry in CHANGES.rst under the corresponding subsection
• [x] updated relevant tests
• [x] updated relevant documentation
• [x] Passed romancal regression testing on Jenkins / PLWishMaster. Link: https://plwishmaster.stsci.edu:8081/job/RT/job/romancal/XXX/

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 60.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 97.18%. Comparing base (087a60d) to head (61d4459). Report is 17 commits behind head on main.

Files	Patch %	Lines
src/roman_datamodels/datamodels/_datamodels.py	60.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #348 +/- ## ========================================== - Coverage 97.56% 97.18% -0.38% ========================================== Files 30 31 +1 Lines 2788 3059 +271 ========================================== + Hits 2720 2973 +253 - Misses 68 86 +18 ```

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[PaulHuwe]

Can you please add a regression test link?

[stscieisenhamer]

regression test

[stscieisenhamer]

New regression test