Issue/12525 custom field creation

[hichamboushaba]

Closes: #12525 Closes: #12532

Description

This PR adds the following:

• Entry point to add new custom fields.
• Validation to ensure we are not adding duplicate keys when creating new fields.
• Validation to ensure we are not allowing keys that start with _.

Steps to reproduce

1. Open an order that has at least one custom field (can be added in wp-admin from the Custom Fields section, later we'll enable showing the button even when the order doesn't have any fields).
2. Tap on View Custom Fields section.
3. Tap on the Add button.

Testing information

• Test the happy path of adding a field and saving it.
• Test adding a duplicate key.
• Test adding/modifying a key to start with _

The tests that have been performed

The above.

Images/gif

Screen_recording_20240905_162925.mp4

• I have considered if this change warrants release notes and have added them to RELEASE-NOTES.txt if necessary. Use the "[Internal]" label for non-user-facing changes.

Reviewer (or Author, in the case of optional code reviews):

Please make sure these conditions are met before approving the PR, or request changes if the PR needs improvement:

• The PR is small and has a clear, single focus, or a valid explanation is provided in the description. If needed, please request to split it into smaller PRs.
• Ensure Adequate Unit Test Coverage: The changes are reasonably covered by unit tests or an explanation is provided in the PR description.
• Manual Testing: The author listed all the tests they ran, including smoke tests when needed (e.g., for refactorings). The reviewer confirmed that the PR works as expected on big (tablet) and small (phone) in case of UI changes, and no regressions are added.

[dangermattic]

	1 Warning
⚠️	This PR is assigned to the milestone 20.4. This milestone is due in less than 2 days. Please make sure to get it merged by then or assign it to a milestone with a later deadline.

Generated by 🚫 Danger

[wpmobilebot]

📲 You can test the changes from this Pull Request in WooCommerce-Wear Android by scanning the QR code below to install the corresponding build.

	App Name	WooCommerce-Wear Android
	Platform	⌚️ Wear OS
	Flavor	Jalapeno
	Build Type	Debug
	Commit	9673681
	Direct Download	woocommerce-wear-prototype-build-pr12531-9673681.apk

[wpmobilebot]

📲 You can test the changes from this Pull Request in WooCommerce Android by scanning the QR code below to install the corresponding build.

	App Name	WooCommerce Android
	Platform	📱 Mobile
	Flavor	Jalapeno
	Build Type	Debug
	Commit	9673681
	Direct Download	woocommerce-prototype-build-pr12531-9673681.apk

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 82.75862% with 5 lines in your changes missing coverage. Please review.

Project coverage is 40.62%. Comparing base (0ca000b) to head (9673681).
Report is 10 commits behind head on trunk.

Files with missing lines	Patch %	Lines
...customfields/editor/CustomFieldsEditorViewModel.kt	83.33%	1 Missing and 3 partials ⚠️
.../android/ui/customfields/CustomFieldsRepository.kt	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk   #12531      +/-   ##
============================================
+ Coverage     40.61%   40.62%   +0.01%     
- Complexity     5662     5665       +3     
============================================
  Files          1228     1228              
  Lines         68879    68901      +22     
  Branches       9541     9543       +2     
============================================
+ Hits          27972    27991      +19     
- Misses        38336    38338       +2     
- Partials       2571     2572       +1     

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[irfano]

Good job! I didn't encounter any issues. 👍🏻 LGTM!