fix(metastore-cache): import dao in methods

[villebro]

SUMMARY

The recent PR #29344 caused a regression causing importing of the Metastore cache outside the app context to fail. This moves the DAO import into the relevant methods to avoid needing the app context during import time and also changes the imports in the test to ensure they work without an app context.

AFTER

python
Python 3.10.6+ (heads/3.10:57f4472, Aug 30 2022, 12:15:19) [Clang 13.1.6 (clang-1316.0.21.2.5)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from superset.extensions.metastore_cache import SupersetMetastoreCache
>>>

BEFORE

$ python
Python 3.10.6+ (heads/3.10:57f4472, Aug 30 2022, 12:15:19) [Clang 13.1.6 (clang-1316.0.21.2.5)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from superset.extensions.metastore_cache import SupersetMetastoreCache
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/ville/apple/apache-superset/superset/extensions/metastore_cache.py", line 27, in <module>
    from superset.daos.key_value import KeyValueDAO
  File "/Users/ville/apple/apache-superset/superset/daos/key_value.py", line 32, in <module>
    from superset.key_value.models import KeyValueEntry
  File "/Users/ville/apple/apache-superset/superset/key_value/models.py", line 24, in <module>
    from superset.models.helpers import AuditMixinNullable, ImportExportMixin
  File "/Users/ville/apple/apache-superset/superset/models/__init__.py", line 17, in <module>
    from . import core, dynamic_plugins, sql_lab, user_attributes  # noqa: F401
  File "/Users/ville/apple/apache-superset/superset/models/core.py", line 74, in <module>
    from superset.models.helpers import AuditMixinNullable, ImportExportMixin
  File "/Users/ville/apple/apache-superset/superset/models/helpers.py", line 107, in <module>
    config = app.config
  File "/Users/ville/apple/apache-superset/venv/lib/python3.10/site-packages/werkzeug/local.py", line 318, in __get__
    obj = instance._get_current_object()
  File "/Users/ville/apple/apache-superset/venv/lib/python3.10/site-packages/werkzeug/local.py", line 519, in _get_current_object
    raise RuntimeError(unbound_message) from None
RuntimeError: Working outside of application context.

This typically means that you attempted to use functionality that needed
the current application. To solve this, set up an application context
with app.app_context(). See the documentation for more information.
>>>

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.87%. Comparing base (76d897e) to head (481bc7b).
Report is 403 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #29451       +/-   ##
===========================================
+ Coverage   60.48%   83.87%   +23.38%     
===========================================
  Files        1931      519     -1412     
  Lines       76236    37412    -38824     
  Branches     8568        0     -8568     
===========================================
- Hits        46114    31379    -14735     
+ Misses      28017     6033    -21984     
+ Partials     2105        0     -2105     

Flag	Coverage Δ
hive	49.15% <0.00%> (-0.01%)	⬇️
javascript	?
mysql	77.16% <100.00%> (?)
postgres	77.26% <100.00%> (?)
presto	53.78% <0.00%> (-0.02%)	⬇️
python	83.87% <100.00%> (+20.38%)	⬆️
sqlite	76.74% <100.00%> (?)
unit	59.76% <0.00%> (+2.13%)	⬆️

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[rtexelm]

LGTM, appreciate the fix!