Add WordPress multisite support (network-wide credentials storage) #44

Currently, AI provider credentials are stored as a site option via get_option( 'wp_ai_client_provider_credentials', [] ), and the credentials admin page is registered via add_options_page() in wp-ai-client.

In WordPress multisite, this means using the same provider API keys across the network requires entering/storing credentials separately on each site. In some cases that’s fine, but for many multisite networks (and host-managed setups) a single network-level “source of truth” for credentials is preferable.

This would allow a network-activated plugin (or a host) to manage provider keys once and use them throughout the multisite network without custom syncing code.

Feature Request:

• Add supported multisite behavior for credentials storage, e.g. an opt-in mode to read/write credentials using get_site_option() / update_site_option() (network-wide) instead of per-site options.
• Ideally provide a hook/filter to customize the credentials storage mechanism so ecosystems can integrate without mirroring options across sites.