omit_defaults supports specifying tuple/frozenset default values via default_factory

[jcrist]

Previously omit_defaults would only detect tuple/frozenset fields containing default values if the default was specified by value.

class Ex(msgspec.Struct, omit_defaults=True):
    x: tuple = ()                                    # omit_defaults would work properly with this spelling
    y: tuple = msgspec.field(default=())             # or with this spelling
    z: tuple = msgspec.field(default_factory=tuple)  # but not with this spelling

We now also support if these types are specified via default_factory (all 3 cases above now properly work with omit_defaults=True).

Note that since tuple and frozenset values are immutable there's no real reason to prefer default_factory for defaults of these types.

Fixes #652.