Allow mapping of a Google ID without a prefix

[mikkamp]

Changes proposed in this Pull Request:

This PR makes the prefix when fetching status from the Merchant Center as optional. We do want to make sure that the ID is a strictly numeric ID otherwise it's not accepted. In the previous regex we searched till end of string with $, but didn't force it to search from the start of the string with ^. This meant that it was possible to a map an ID like foo_gla_12345. We fixed this in the PR to ensure it scans the whole string for a matching pattern:

• gla_ + numeric ID
• numeric ID

Any other patterns will result in an unmapped ID. I've added additional unit tests to cover all these variations.

Closes #2768

Detailed test instructions:

1. Add the snippet from the documentation: https://woocommerce.com/document/google-listings-and-ads/faqs/#how-do-i-remove-the-gla-prefix-from-synced-products
2. Setup the extension to sync some products to Merchant Center
3. Use the Connection Test helper to clear the product statuses
4. Refresh the product feed page
5. Wait for the statuses to reload and confirm that we have products that are a different status then unsynced

Changelog entry

• Tweak - Allow mapping of a Google ID without a prefix.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.2%. Comparing base (9949c75) to head (a9dbb55).

Additional details and impacted files

@@            Coverage Diff            @@
##             develop   #2773   +/-   ##
=========================================
  Coverage       67.2%   67.2%           
  Complexity      4677    4677           
=========================================
  Files            480     480           
  Lines          19571   19571           
=========================================
  Hits           13159   13159           
  Misses          6412    6412           

Flag	Coverage Δ
php-unit-tests	67.2% <100.0%> (ø)

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Files with missing lines	Coverage Δ
src/Product/ProductHelper.php	92.3% <100.0%> (ø)