Add plugin ability to defer to default type analyzer

[kcdodd]

• The essence of this PR is to allow a Plugin's callback for get_type_analyze_hook to defer back to the default implementation.
• The motivating use-case that lead to needing this was specifically to help customizing type analysis for a variadic generic class which could be used in contexts (like inheritance) where the arguments would be an unpacked TypeVarTuple. The mechanisms for dealing with this seem complicated, and not exposed via the plugin API (but I could be mistaken on that). As far as I could tell it requires specialized logic for enabling/handling the unpacking etc, and besides these are not the instances that need to be customized in this case.
• The approach here is to return NotImplemented as a sentinel to indicate the intention. This appears to not require altering the annotated return Type (but I'm not familiar with why that is the case), but it seems that for example returning None would require the callback signature to change, which might be an irritation. Also, in my personal experience this is preferable to raising say NotImplementError since the intent can be less clear as to where exactly the error was raised compared to a return value.
• The included change appears to be the only place where the hook is called, and seems to not alter the succeeding logic if it was simply ignored.
• I am somewhat new to the mypy source style, so I apologize if this PR doesn't conform to conventions.

Following is an example of the use-case mentioned above...

# example use case
def _analyze_type(ctx: AnalyzeTypeContext) -> Type:
  for arg in ctx.type.args:
    if isinstance(arg, UnboundType) and arg.name == 'Unpack':
      return NotImplemented
    ...

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅