Support `notifications/resources/updated`

[LucaButBoring]

Expected Behavior

Support receiving data from individual resource subscriptions (notifications/resources/updated) in SyncSpec/AsyncSpec. Currently, only resource list subscriptions (notifications/resources/list_changed) are supported.

Current Behavior

notifications/resources/updated is not currently supported. Notably, there is already a method to subscribe to an individual resource (McpClient#subscribeResource(SubscribeRequest)), but there is no way to register an appropriate client handler for it.

Context

Specification: https://modelcontextprotocol.io/specification/2025-03-26/server/resources#subscriptions