[Simple Payments]: Do not duplicate productId on block duplication

[stacimc]

This PR is one possible solution to #12801. It relies on the addition of support for unique block attributes proposed in this (currently open) Gutenberg PR: WordPress/gutenberg#32604.

Fixes #12801

Changes proposed in this Pull Request:

When a Simple Payments block is duplicated, all of its attributes are copied including the productId. This causes the duplicated block to point to the same product resource under the covers -- so if the user then edits the duplicate and saves the post, the edits will (confusingly) be applied to both blocks. This PR resolves the problem by marking the productId attribute of the Simple Payments block as unique so that it is not copied on block duplication.

Now when the block is duplicated, all of its fields (price, title, description, etc) will be copied, but a new product is created on the backend. Edits to each block can be made individually.

Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

• Pull this Gutenberg PR: [Block Attributes]: Add support for non-duplicable block attributes WordPress/gutenberg#32604.
• Pull this branch in Jetpack
• Sync both Gutenberg and Jetpack to your sandbox
• Create a post and insert a Pay with Paypal block. Fill out all of its fields.
• Duplicate the block. The new block should look identical to the first.
• Optionally, view the blocks in the Code editor to see their attributes. Verify that they have different values for productId.
• Edit the fields of the second block.
• Save the post.
• Refresh the page; your edits should only be applied to the second block. View the post on the frontend and verify that this true on the frontend as well.

[matticbot]

Caution: This PR has changes that must be merged to WordPress.com
Hello stacimc! These changes need to be synced to WordPress.com - If you 're an a11n, please commandeer and confirm D62724-code works as expected before merging this PR. Once this PR is merged, please commit the changes to WP.com. Thank you!
This revision will be updated with each commit to this PR

[github-actions]

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[jeherve]

Closing this for now because of the lack of activity on this. We can always reopen in the future if needed.