Allow configuring the cookie domain for _fbp/_fbc

[loevgaard]

Problem

Meta's pixel writes _fbp and _fbc on the registrable domain (for example .example.com) and encodes the level it used in the second segment of the value: fb.1. means example.com, fb.2. means www.example.com, fb.0. means com.

StoreFbpSubscriber and StoreFbcSubscriber create host-only cookies (no domain argument to Cookie::create()), while the SDK value objects default the subdomain index to 1. On a site served from www.example.com the result is a host-only cookie whose value claims to be a domain cookie. When a visitor moves between www.example.com, example.com or a checkout subdomain, the browser presents different cookies, and the browser pixel may write its own domain cookie next to the server one, producing two _fbp values for one person.

Suggested fix

Add configuration:

setono_meta_conversions_api:
    cookies:
        domain: null          # e.g. '.example.com'; null = host-only
        lifetime: '+90 days'

Set the subdomain index on generated Fbp/Fbc values accordingly (withSubdomainIndex(2) for host-only on a www. host, 1 when a domain cookie is configured). Meta's capi-param-builder ships an ETLDPlus1Resolver that shows the intended logic if auto-detection is preferred over configuration.

Related: #28.