Add optional `first_name` to `Spree::Address` behind a configuration flag

Background

Solidus 3.0 collapsed firstname/lastname on Spree::Address into a single name column (#3234). That remains the right default, but some stores (notably in EU markets, see #6110) need a separate given name for e-invoicing and for third-party services that require split names. Some stores use solidus_address_name or your their own customizations to extract names from addresses.

#6159 attempted to bring split names back into core, and the discussion there produced a design the core team agreed on. That PR has gone stale, so this issue captures the agreed design so a fresh PR can be written against it.

Agreed upon design

• Reintroduce first_name only. The existing name column stays and doubles as the last name when the feature is enabled. We do not reintroduce lastname.
• Control the feature with a Spree::Config preference that defaults to the current unified approach. Stores that continue to use a single name field must see no change in behaviour at all.
• first_name is optional by default. Stores can add a validation to require if need be, but let's all remember the Falsehoods Programmers Believe About Names.
• If a store enables the flag, existing name values are left untouched and first_name is simply empty until filled in.
• Backwards compatible for existing stores. The firstname column still exists in the schema. Whether we reuse it (and un-ignore it) or add a new first_name column can be decided in the PR, but any change must not require action from stores that leave the flag off.

Out of scope

• Any data migration between single and split names. Stores that switch modes are responsible for their own data.

Related

• #6159 (previous attempt, closed in favour of this issue)
• #6110 (compliance issue that motivated it)
• #3234 (original RFC to remove firstname/lastname)

Start by reading the Spree::Address and Spree::Config implementations, then inspect the existing schema and related tests around address names. Implement the agreed configuration flag without changing the default behavior; with the flag enabled, first_name should be optional while existing name values remain untouched, and stores with the flag off should require no action.