Support for serialize_as_any runtime setting

[sydney-runkle]

Closes #6423
Counterpart PR: pydantic/pydantic-core#1194

Requires new pydantic-core release in order to support the new runtime flag 👍. Leaving WIP tag until we release a new pydantic-core version with the necessary changes!

TODO:

• Add support for dataclasses, typed dicts
• Add support for TypeAdapter
• Experiment with how to set this at the project level
• Add a what is duck typing section to the docs
• Add more recursive / nested model testing

Selected Reviewer: @davidhewitt

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[codspeed-hq]

CodSpeed Performance Report

Merging #8830 will not alter performance

_{Comparing ser_as_any_support (3183633) with main (df5be31)}

Summary

✅ 10 untouched benchmarks

[sydney-runkle]

Note, I wanted to experiment a bit with how a user of pydantic could configure the serialize_as_any default within his/her project. The following is one approach to this:

from pydantic import BaseModel, SecretStr

class MyBaseModel(BaseModel):
    def model_dump(self, **kwargs):
        # use super's model_dump, with serialize_as_any set to True by default
        return super().model_dump(serialize_as_any=True, **kwargs)
    
    def model_dump_json(self, **kwargs):
        # use super's model_dump_json, with serialize_as_any set to True by default
        return super().model_dump_json(serialize_as_any=True, **kwargs)


class User(BaseModel):
    name: str

class UserInfo(User):
    password: SecretStr

class OuterModel(MyBaseModel):
    user: User

u = OuterModel(user=UserInfo(name='John', password='secret_pw'))
print(u.model_dump_json())
#> {"user":{"name":"John","password":"**********"}}

[sydney-runkle]

Please review

[dmontagu]

Looks good other than the docs suggestions (which you can take or leave), assuming the pydantic-core PR is good etc.