Store: Only send back modified values back to the payment method API

[justinshreve]

Fixes #21670.

This PR modifies the action list code to only send back modified values to the REST API, instead of the full payment method object.

It also clears out updates state from our edits object on successful save (so that you can make further changes without passing back existing edits over the API) like we do in other places like products state.

To Test:

• Run npm run test-client client/extensions/woocommerce/state and make sure all tests pass.
• Go to http://calypso.localhost:3000/store/settings/payments/:store
• Modify various payment methods. This includes titles, descriptions, settings (like BACs accounts), and toggling enabled/disabled for various payment methods. Make sure after saving, all values are displayed correctly.
• Do a hard refresh to verify your settings saved correctly and that there was no data loss.
• Do the above once more with different settings to make sure everything is 💯 .

[matticbot]

Test live: https://calypso.live/?branch=update/payment-settings

[timmyc]

Testing out well for me, thanks for making this change 🌟