Context
After WT-16052 and WT-16053, we should have an extension example module and the ability to read/write to the turtle file. As such, we now implement the write KEK mechanism. During startup, or checkpoint pick up, we need the ability to read the current KEK blob.

Please follow Steps for startup, Steps for checkpoint pickup on where we need perform the read path.

Test Plan
Leader/Follower mode
Startup (on fresh WT): New KEK blob is written
KEK rotation -> crash: KEK blob is not persisted
KEK rotation -> checkpoint: KEK blob should be persisted

Defintion of Done

• WiredTiger utilises get_key_blob within the startup, and checkpoint pick up.
• Passes current key blob back to encryption module via load_key_blob
• Tests scenarios are created using either example, or mock module