Verify partial-exit blob signature on POST in validateExitBlob

[LukeHackett12]

Implement signature verification in the validateExitBlob endpoint so that when a partial-exit blob is submitted via POST, the signature is checked to ensure it was signed by the claimed operator. This will prevent invalid or spoofed blobs from being accepted.

• validateExitBlob can also expect an ENR In addition to API params/payload so that solo flow is also covered.