[SiOC - customer creation] Customer search: CTA and integration with existing customer selector

Closes: #12652

Why

Another way to add/edit a customer in order form other than entering the email/addresses manually is to select an existing customer. This is already supported in the app, and this PR creates a CTA based on the design and integrates with the existing customer search flow.

How

The CTA is located above the customer card, at the trailing edge of the customer section header text in OrderCustomerSection. When the search icon is tapped, it opens a sheet of the customer selector under the viewModel.showsCustomerSearch boolean. The customer selector requires a addressFormViewModel: CreateOrderAddressFormViewModel parameter, even though it isn't used in the selector internally because we disallow editing customer details by disallowCreatingCustomer: true.

While testing, I encountered a bug where selecting a customer from the selector sheet (where dismiss is called in the view controller) would close both the sheet and the order form. It turned out that because I originally reinitialized the OrderCustomerSectionViewModel in configureCustomerDataViewModel whenever the order changes, the section view model's showsCustomerSearch boolean also got reset to false (default value), and thus the sheet is hidden automatically and order form got dismissed instead. To fix this issue, I updated OrderCustomerSectionViewModel to have the same lifetime as EditableOrderViewModel / order form and synced the customer data via a published variable.

Testing instructions

[x] @jaclync tests the production flow when the feature flag is disabled

Prerequisite: the store has at least two different customers

Create order

Go to the orders tab
Tap +
Tap Add Customer --> there should be new space above the customer card with a search icon at the trailing edge
Tap on the search icon --> it should show the customer selector sheet
Tap on a customer (guest or customer with an account) --> the customer details should be filled into the customer card
Tap Create --> the customer details should be saved to the new order remotely

Edit order

Go to the orders tab
Tap on an existing order with a customer that has an email set
Tap Edit --> a customer card should be shown
Tap on the search icon --> it should show the customer selector sheet
Tap on a different customer (guest or customer with an account) --> the customer details should be filled into the customer card, and the order's customer details should be saved remotely after the spinner disappears

Screenshots

https://github.com/woocommerce/woocommerce-ios/assets/1945542/3b777ec4-d9ac-4d4f-a54b-6abc5d087122

[x] I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

	App Name	WooCommerce iOS
	Build Number	pr12654-036c381
	Version	18.5
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	036c381f435a3148a39a1a99f44bfae93439d9c8
	App Center Build	WooCommerce - Prototype Builds #8960

woocommerce / woocommerce-ios