fix: support custom root type with nested models in parse_obj

[PrettyWood]

Change Summary

Allow to parse nested models with __root__. To do that we can do a check in the __init__ function or simply ensure __root__ key is present when needed in validate.
The latter option has been chosen.

Related issue number

closes #1193
closes #2100

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• changes/<pull request or issue id>-<github username>.md file added describing change
  (see changes/README.md for details)

[codecov]

Codecov Report

Merging #2238 (4e8bf4d) into master (13a5c7d) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master     #2238   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           21        21           
  Lines         4199      4199           
  Branches       854       854           
=========================================
  Hits          4199      4199           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 13a5c7d...4e8bf4d. Read the comment docs.