Cannot serialize _CachedRelation

[jtcohen6]

Reproduction steps

Run test__postgres__simple_copy_with_materialized_views with JSON-formatted logging:

DBT_LOG_FORMAT=json python3 -m pytest -m profile_postgres test/integration/001_simple_copy_test/test_simple_copy.py::TestSimpleCopy::test__postgres__simple_copy_with_materialized_views 

core/dbt/events/functions.py:208: in create_log_line
    return create_json_log_line(e, msg_fn)  # json output, both console and file
core/dbt/events/functions.py:200: in create_json_log_line
    values = event_to_serializable_dict(e, lambda _: e.get_ts_rfc3339(), lambda x: msg_fn(x))
core/dbt/events/functions.py:139: in event_to_serializable_dict
    for field, value in dataclasses.asdict(e).items():  # type: ignore[attr-defined]
/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.8/lib/python3.8/dataclasses.py:1073: in asdict
    return _asdict_inner(obj, dict_factory)
/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.8/lib/python3.8/dataclasses.py:1080: in _asdict_inner
    value = _asdict_inner(getattr(obj, f.name), dict_factory)
/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.8/lib/python3.8/dataclasses.py:1114: in _asdict_inner
    return copy.deepcopy(obj)
/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.8/lib/python3.8/copy.py:153: in deepcopy
    y = copier(memo)
core/dbt/adapters/cache.py:82: in __deepcopy__
    new = self.__class__(self.inner.incorporate())
core/dbt/contracts/relation.py:51: in incorporate
    value = self.to_dict(omit_none=True)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <PostgresRelation "dbt"."test16384599476280903696_simple_copy_001"."unrelated_materialized_view">, use_bytes = False, use_enum = False
use_datetime = False

>   ???
E   AttributeError: 'str' object has no attribute '_serialize'

<string>:14: AttributeError

In particular, the AddRelation event (included because --log-cache-events is enabled) has a _CachedRelation object as a dataclass property. This seems like the same issue we were observing in #4361 (comment):

ipdb> dataclasses.asdict(e)
*** AttributeError: 'str' object has no attribute '_serialize'
ipdb> e.__dataclass_fields__
{'relation': Field(name='relation',type=<class 'dbt.events.stubs._CachedRelation'>,default=<dataclasses._MISSING_TYPE object at 0x10a541640>,default_factory=<dataclasses._MISSING_TYPE object at 0x10a541640>,init=True,repr=True,hash=None,compare=True,metadata=mappingproxy({}),_field_type=_FIELD), 'code': Field(name='code',type=<class 'str'>,default='E024',default_factory=<dataclasses._MISSING_TYPE object at 0x10a541640>,init=True,repr=True,hash=None,compare=True,metadata=mappingproxy({}),_field_type=_FIELD)}