Add stats emails summary endpoint

[adalpari]

Description

Add three new WP.com API endpoints to support the Subscribers tab feature:

1. Stats emails summary — /rest/v1.1/sites/{site_id}/stats/emails/summary
2. Stats subscribers — /rest/v1.1/sites/{site_id}/stats/subscribers
3. Subscribers by user type — /wpcom/v2/sites/{site_id}/subscribers_by_user_type

Changes

Stats emails summary

• Add StatsEmailsSummaryParams with period, quantity, sort_field, and sort_order parameters
• Add StatsEmailsSummaryResponse and StatsEmailsSummaryPost response types
• Add endpoint definition using WpDerivedRequest macro with WpComNamespace::RestV1_1
• Wire up the endpoint in WpComApiRequestBuilder and WpComApiClient
• Add 10 unit tests and 3 JSON test fixtures
• Add e2e integration test

Stats subscribers

• Add StatsSubscribersParams with unit, quantity, and stat_fields (comma-joined) parameters
• Add StatsSubscribersResponse reusing StatsVisitsDataValue for the fields+data array pattern
• Add helper methods subscribers_data() and subscribers_paid_data() with #[uniffi::export]
• Add endpoint definition with WpComNamespace::RestV1_1
• Wire up the endpoint in WpComApiRequestBuilder and WpComApiClient
• Add 13 unit tests and 5 JSON test fixtures (day/week/month/year/empty)
• Add e2e integration test

Subscribers by user type

• Add SubscribersByUserTypeParams with user_type, per_page, page, and sort parameters
• Add SubscribersByUserTypeUserType and SubscribersByUserTypeSortField enums
• Reuse existing ListSubscribersResponse type
• Add ListSubscribersByUserType variant to SubscribersRequest with WpComNamespace::V2
• Add 4 unit tests and 2 JSON test fixtures

Test plan

• cargo build succeeds
• cargo test --lib -p wp_api — all new unit tests pass
• cargo fmt clean
• Run e2e tests with WP.com credentials

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