Correctly set required and nullable json schema values

[martinostvik]

When using this library on models that has a json_schema_extra field populated with "nullable" False etc the resulting model will not be marked as nullable in the json schema.

pydantic-partial should mark all fields as optional in the resulting partial model.
An example is having a pydantic model with a field that has a default value.
In API responses you want the field to not be nullable (as it has a default value, it will not be null in the API response).
To acchieve this you can create a model like this:

from pydantic import BaseModel, Field

class Pet(BaseModel):
    name: str = Field(default="Charlie", json_schema_extra={"required": True})

This will make sure the field name has a default value if not provided, AND that the json schema for the API response iwll not mark it as nullable.

But for a PATCH request etc you can use pydantic-partial to create a version of this model where all fields are optional for the request body.
The issue is that this library won't override the json_schema_extra values already existing on the model.

from pydantic import BaseModel, Field
from pydantic_partial import create_partial_model

class A(BaseModel):
    name: str = Field(default="Bob", json_schema_extra={"required": True})

class B(BaseModel):
    name: str = Field(default="Bob", json_schema_extra={"nullable": False})

class C(BaseModel):
    name: str = Field(default="Bob")
    
PartialA = create_partial_model(A)
PartialB = create_partial_model(B)
PartialC = create_partial_model(C)

print(PartialA.model_json_schema())
print(PartialB.model_json_schema())
print(PartialC.model_json_schema())

Results before my changes:
Partial A:

{
   "properties":{
      "name":{
         "default":"Bob",
         "required":true,  <---- required and not nullable
         "title":"Name",
         "type":"string"
      }
   },
   "title":"A",
   "type":"object"
}

PartialB:

{
   "properties":{
      "name":{
         "default":"Bob",
         "nullable":false, <--- Not nullable
         "title":"Name",
         "type":"string"
      }
   },
   "title":"B",
   "type":"object"
}

PartialC: (Has a default value, and no "required" or "nullable" values. Means it is nullable)

{
   "properties":{
      "name":{
         "default":"Bob",
         "title":"Name",
         "type":"string"
      }
   },
   "title":"C",
   "type":"object"
}

Results after the changes in this PR:

PartialA:

{
   "properties":{
      "name":{
         "anyOf":[
            {
               "type":"string"
            },
            {
               "type":"null"
            }
         ],
         "default":"None",
         "nullable":true,   <---- nullable ✅
         "required":false,  <---- not required ✅
         "title":"Name"
      }
   },
   "title":"APartial",
   "type":"object"
}

PartialB:

{
   "properties":{
      "name":{
         "default":"Bob",
         "nullable":false, <---- nullable ✅
         "title":"Name",
         "type":"string"
      }
   },
   "title":"B",
   "type":"object"
}

PartialC: (Has a default value, and no "required" or "nullable" values. Means it is nullable)

{
   "properties":{
      "name":{
         "default":"Bob",
         "title":"Name",
         "type":"string"
      }
   },
   "title":"C",
   "type":"object"
}