Add support for multiple flatten enums

[rlebran]

Update #264 following v1 merge on master.

Fix #165.

Use all_of when flattening multiple subschemas.

There is still a limitation when flattening required enum. We lose the information that the field is required. This is not caused by this PR. This could be fix in another PR.

We use this fix in apistos.

[GREsau]

Thanks for the PR! I amended it so that it no longer overwrites the allOf in the original schema. Otherwise, when flattening more than 3 enums, the 3rd and 4th would overwrite the 1st and 2nd

[rlebran]

Thank you so much for the work and the merge !