Add union type and subtypes check in schema model signature

[fazeelghafoor]

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Fixes #1153 Add support for union type field in model schema

[fazeelghafoor]

@vitalik can you please review and let me know if anything needs to be modified. I think this is a partial fix to #1153 but I felt like at least add support for UnionTypes.

[vitalik]

Could you also add some unit tests for this ?

[Mapiarz]

@fazeelghafoor Thanks for this fix! Any chance to see the extra coverage on this? It's a pity this fix is stuck and it's really important imho.

[dan-blanchard]

I tried to pick this up in https://github.com/vitalik/django-ninja/pull/1512