feat(identity): switch from libsecp256k1 to k256

`libsecp256k1` is considered unmaintained, see [#159](paritytech/libsecp256k1#159).
Brief discussion here: #5888.
[`k256`](https://crates.io/crates/k256) is also a pure Rust implementation just like `libsecp256k1`.

Pull-Request: #5892.

Author: drHuangMHT