Restructure the order transaction state tree.

[southp]

Summary

This is a follow-up task of making the best use of the newly-deployed order transaction status endpoint, following #23792 . It seems big, but it actually can be boiled down into:

1. Data layer
   1. Enhance fromApi() to camelCase the keys and convert the processing status into a set of known constants.
2. State tree
   1. Add isFetching and errors substate under orderTransactions
   2. A new action for dispatching errors.
   3. Add selectors for the above new substates.
   4. Add new test cases for covering them.

Test Plan

1. Run npm run test-client order-transactions
2. Run npm run test-client "transactions/order"

[matticbot]

Test live: https://calypso.live/?branch=update/match-response-format-of-order-transaction-endpoint

[southp]

Hmm, I've found a weird bug when doing a manual e2e test.
When GET /me/transactions/order/:order_id endpoint responds with a 200 response, e.g.

{order_id: 150077, user_id: 107809150, processing_status: "processing"}

Somehow the action passed to https://github.com/Automattic/wp-calypso/blob/master/client/state/data-layer/wpcom-http/utils.js#L382 has meta.dataLayer like this:

It makes the following getData() to return '' and make fetchAction to https://github.com/Automattic/wp-calypso/blob/master/client/state/data-layer/wpcom-http/utils.js#L410. Since the fetchAction here is just yet another ORDER_TRANSACTION_FETCH, the whole flow will repeat again and again.

Normally meta.dataLater.data should be filled with the response data and meta.dataLayer.headers is a object containing the parsed header.

[southp]

@Automattic/payments @Automattic/i18n I've resolved the above issue in 1365c79. This PR should now work and is ready for review 🙇🏻

[yoavf]

I don't feel equipped to fully review the state code, but overall:

• Code LGTM
• Works as expected
• Tests cover all changes
• No apparent regression in existing code
• Tested e2e as part of Payments: Add receipt id to the route after a successful redirect checkout. #24054 and Implement the pending checkout page. #23715