feat: Add ec.totals.change to Embedded Checkout Protocol

[lemonmade]

Description

This PR adds an ec.totals.change message to the Embedded Checkout Protocol. This message allows hosts to be informed of changes to the checkout state that do not have an accompanying ec.{domain}.change event today, like taxes and fees, which are entirely contained in totals. It also allows hosts to be informed of totals-only changes that may result from asynchronous updates after a domain event, such as potentially-slow shipping rate updates after a ec.fulfillment.change message has indicated the initial application of the change.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing
  functionality to not work as expected, including removal of schema files
  or fields)
• Documentation update

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Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[westeezy]

I think this seems reasonable and appreciate the strict rule on ordering to keep it predictable.

[jingyli]

Change overall LGTM, just have a conceptual question regarding expectation when a domain-specific notification exists.

[jingyli]

Naive QQ: I understand this is nice from a consistency perspective, but is there any additional benefits from mandating business to MUST send both notifications if the domain-specific message also has its own mandate to send the entire latest checkout state (which naturally encapsulates ec.totals.change)? It feels like we are going to be exchanging the same checkout object multiple times throughout the embedded checkout's lifecycle (especially if we are going to mirror this pattern for other EP bindings like Embedded Cart, where it's possible many line_item changes will occur).

Should we loosen the guideline to MAY (or SHOULD) or do the opposite where if a domain-specific message exist, then that must be the one message sent back to host with the updated totals?

[lemonmade]

We discussed this internally too, as were also a little worried by the amount of data flying over the wire for these changes. In the end, I think this contract is still right, though — it is weird for only some changes to totals to be communicated with this message, but not others, and I felt uncomfortable with the implementation instructions for both host and embedded checkout when I tried writing docs for a version that deduplicated ec.totals.change with other messages.

[mmohades]

Thanks for putting this together @lemonmade !! Beautifully done! :)