feat(datastore): Add isLoaded public property in List+Model

[thisisabhash]

Issue #

#3286

Description

This public API change adds a isLoaded property in the List+Model public to help customers check if the list is loaded before calling fetch() to avoid network calls.

General Checklist

• Added new tests to cover change, if needed
• Build succeeds with all target using Swift Package Manager
• All unit tests pass
• All integration tests pass
• Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)
  - [ ] Documentation update for the change if required
• PR title conforms to conventional commit style
  - [ ] New or updated tests include Given When Then inline code documentation and are named accordingly testThing_condition_expectation()
  - [ ] If breaking change, documentation/changelog update with migration instructions

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Merging #3296 (629b91f) into main (ec5a64d) will decrease coverage by 0.18%.
The diff coverage is n/a.

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@@            Coverage Diff             @@
##             main    #3296      +/-   ##
==========================================
- Coverage   64.50%   64.33%   -0.18%     
==========================================
  Files        1075     1075              
  Lines       35880    35880              
==========================================
- Hits        23146    23082      -64     
- Misses      12734    12798      +64     

Flag	Coverage Δ
API_plugin_unit_test	67.07% <ø> (ø)
AWSPluginsCore	66.15% <ø> (ø)
Amplify	47.84% <ø> (ø)
Analytics_plugin_unit_test	81.37% <ø> (ø)
Auth_plugin_unit_test	74.12% <ø> (+0.02%)	⬆️
CoreMLPredictions_plugin_unit_test	59.44% <ø> (ø)
DataStore_plugin_unit_test	80.07% <ø> (-1.03%)	⬇️
Geo_plugin_unit_test	70.75% <ø> (ø)
Logging_plugin_unit_test	63.09% <ø> (ø)
Predictions_plugin_unit_test	37.29% <ø> (ø)
PushNotifications_plugin_unit_test	60.29% <ø> (ø)
Storage_plugin_unit_test	54.24% <ø> (ø)
unit_tests	64.33% <ø> (-0.18%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...y/Categories/DataStore/Model/Lazy/List+Model.swift	76.66% <ø> (ø)

... and 11 files with indirect coverage changes

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