text: Title: Execution contexts missing some functions
  Body: SQLAlchemy allows defining column defaults with a callable getting the insert/update context as an argument ([doc](https://docs.sqlalchemy.org/en/13/core/defaults.html?highlight=column%20default%20callable#context-sensitive-default-functions)), e.g:
  
  ```python
  def get_default(context):
      return context.get_current_parameters()['name'] + 'whatever'
  
  
  class MyModel(Base):
      __tablename__ = 'my_model'
  
      id = Column(Integer, primary_key=True)
      name = Column(Unicode, nullable=False)
      something = Column(Unicode, default=get_default, nullable=False)
  ```
  
  I'm trying to add type annotations to the `get_default` function in my code. For the example above, the return type would be a `str` (the column is defined as `Unicode`).
  
  The `context` argument is (in my case, using PostgreSQL with the pyscopg2 backend) an instance of the `sqlalchemy.dialects.postgresql.psycopg2.PGExecutionContext_psycopg2` class. Its MRO is:
  
  ```
  >>> sqlalchemy.dialects.postgresql.psycopg2.PGExecutionContext_psycopg2.__mro__
  (<class 'sqlalchemy.dialects.postgresql.psycopg2.PGExecutionContext_psycopg2'>,
   <class 'sqlalchemy.dialects.postgresql.base.PGExecutionContext'>,
   <class 'sqlalchemy.engine.default.DefaultExecutionContext'>,
   <class 'sqlalchemy.engine.interfaces.ExecutionContext'>,
   <class 'object'>)
  ```
  
  The problem is none of these classes have the `get_current_parameters()` method defined in sqlalchemy-stubs.
  
  In SQLAlchemy, it's defined in [`sqlalchemy.engine.default.DefaultExecutionContext`](https://github.com/sqlalchemy/sqlalchemy/blob/master/lib/sqlalchemy/engine/default.py#L1324), all the child classes just inherit from it.
  
  I'd be happy to send a pull request adding the stubs for this function, but I'm unsure about the return value, since it returns a dictionary:
  
  > which includes entries for each column/value pair that is part
  > of the INSERT or UPDATE statement. The keys of the dictionary will be
  > the key value of each :class:`.Column`, which is usually synonymous
  > with the name.
  
  So it seems to me the return value would be a `Dict[str, Any]`, since the key (name) of the `Column` will be a string, and its type can be anything, depending on how the column is defined.
  
  ```
  def get_current_parameters(isolate_multiinsert_groups: bool = True) -> Dict[str, Any]: ...
  ```
  
  Is that correct?