Expose class to get_tag hook

[dprbook]

I had a use case, where I needed the fully qualified name of the class in the get_tag hook.
This change calls the hook function with the class as second, optional param, making this change non-breaking.

Alternatively, the second param could be the module path, but this is more flexible.

Usage

from msgspec import Struct

def get_tag(qualname, cls):
  # This still uses the qualname extraction in simple_qualname, see:
  # https://github.com/jcrist/msgspec/blob/bb5bc8dc75509ec6a2e6e0f0b3359028e584c651/msgspec/_core.c#L6060
  return f"{cls.__module__}.{qualname}"

# or
def get_tag(_, cls):
  # Native Python behavior
  return f"{cls.__module__}.{cls.__qualname__}"
  
# still works
def get_tag(qualname):
  return qualname

class S(Struct, tag=get_tag):
  a: int = 1

Any feedback welcome. Nice lib :)