text: Title: Convert all fields to optional
  Body: ### First Check
  
  - [X] I added a very descriptive title to this issue.
  - [X] I used the GitHub search to find a similar issue and didn't find it.
  - [X] I searched the SQLModel documentation, with the integrated search.
  - [X] I already searched in Google "How to X in SQLModel" and didn't find any information.
  - [X] I already read and followed all the tutorial in the docs and didn't find an answer.
  - [X] I already checked if it is not related to SQLModel but to [Pydantic](https://github.com/samuelcolvin/pydantic).
  - [X] I already checked if it is not related to SQLModel but to [SQLAlchemy](https://github.com/sqlalchemy/sqlalchemy).
  
  ### Commit to Help
  
  - [X] I commit to help with one of those options 👆
  
  ### Example Code
  
  ```python
  class HeroBase(SQLModel):
      name: str
      secret_name: str
      age: Optional[int] = None
  
  class HeroUpdate(HeroBase, all_optional=True):
      pass
  ```
  
  
  ### Description
  
  Is it possible to add the ability to modify the fields of a base class to convert them to all optional with a parameter like the table=True but in this case all_optional=True? This would help eliminate the duplication when creating a class for making updates.
  
  ### Wanted Solution
  
  Set all base fields to Optional based on keyword argument.
  
  ### Wanted Code
  
  ```python
  class HeroBase(SQLModel):
      name: str
      secret_name: str
      age: Optional[int] = None
  
  class HeroUpdate(HeroBase, all_optional=True):
      pass
  ```
  
  
  ### Alternatives
  
  _No response_
  
  ### Operating System
  
  Linux, Windows
  
  ### Operating System Details
  
  _No response_
  
  ### SQLModel Version
  
  0.0.4
  
  ### Python Version
  
  3.8.5
  
  ### Additional Context
  
  _No response_