Revert k256 removal (#14499)

* Revert "Remove k256 crate from frame-support dependencies (#14452)"

This reverts commit 4d426214af.

* Keep the test

substrate/Cargo.toml

@@ -300,6 +300,7 @@ hash-db = { opt-level = 3 }
 hmac = { opt-level = 3 }
 httparse = { opt-level = 3 }
 integer-sqrt = { opt-level = 3 }
+k256 = { opt-level = 3 }
 keccak = { opt-level = 3 }
 libm = { opt-level = 3 }
 librocksdb-sys = { opt-level = 3 }