Background: The existing key provider API (SPM-4460) uses a pull model where WiredTiger calls get_key on the module at checkpoint time.

The revised design requires a push model where the module calls into WiredTiger via set_key with key data and an associated timestamp. Before any implementation can begin — on either the WiredTiger or server side — the API surface needs to exist: the set_key function pointer on WT_KEY_PROVIDER, the timestamp field on WT_CRYPT_KEYS. Without this API surface, the server-side key provider team cannot begin writing or testing code against the new push model.

Acceptance Criteria:

• WT_KEY_PROVIDER has a set_key function pointer with correct API docs
• Unused timestamp field on WT_CRYPT_KEYs
• Double check whether adding timestamp field will have any downstream impacts with MDB.
• Existing key provider tests still pass

Note: set_key is a stub returning ENOTSUP. Full implementation is in WT-17536.