Connectors: Register Akismet as a default non-AI connector

[jorgefilipecosta]

Summary

• Registers Akismet in the default connector registry as a spam_filtering type.
• Wires plugin metadata to akismet/akismet.php with an is_active callback defaulting to __return_false, so the connector reports inactive until the Akismet plugin overrides it from its own registration.
• Reuses the existing wordpress_api_key option / WPCOM_API_KEY constant for API key authentication, and points users to https://akismet.com/get/ for credentials.

Must be tested with the latest version of akismet.

Test plan

• With Akismet inactive, confirm the connector appears in the registry and reports is_active as false.
• With Akismet active and registering its own is_active callback, confirm the connector reports is_active as true.
• Verify the wordpress_api_key option and WPCOM_API_KEY constant are picked up as credentials.

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