Bugfix: Remove mutable default usage from Keystore model declaration

[priitlatt]

Fixes #282.

Python 3.11 introduced a more strict check for default values of dataclass fields: https://docs.python.org/3/whatsnew/3.11.html#dataclasses

Since codemagic.models.Keystore defined field certificate_attributes as

@dataclass
class Keystore:
    ...
    certificate_attributes: CertificateAttributes = CertificateAttributes()

where CertificateAttributes is dataclass on its own, this caused a ValueError.

Stacktrace

>>> from codemagic.models import Keystore
Traceback (most recent call last):
  File "", line 1, in 
  File "/Users/priit/.pyenv/versions/3.11.0/lib/python3.11/site-packages/codemagic/models/__init__.py", line 12, in 
    from .keystore import Keystore
  File "/Users/priit/.pyenv/versions/3.11.0/lib/python3.11/site-packages/codemagic/models/keystore.py", line 7, in 
    @dataclass
     ^^^^^^^^^
  File "/Users/priit/.pyenv/versions/3.11.0/lib/python3.11/dataclasses.py", line 1221, in dataclass
    return wrap(cls)
           ^^^^^^^^^
  File "/Users/priit/.pyenv/versions/3.11.0/lib/python3.11/dataclasses.py", line 1211, in wrap
    return _process_class(cls, init, repr, eq, order, unsafe_hash,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/priit/.pyenv/versions/3.11.0/lib/python3.11/dataclasses.py", line 959, in _process_class
    cls_fields.append(_get_field(cls, name, type, kw_only))
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/priit/.pyenv/versions/3.11.0/lib/python3.11/dataclasses.py", line 816, in _get_field
    raise ValueError(f'mutable default {type(f.default)} for field '
ValueError: mutable default  for field certificate_attributes is not allowed: use default_factory