core: replace secp256k with k256 in crypto::ecdsa (#3525)

This PR replaces the usage of
[secp256k](https://crates.io/crates/secp256k1) crate with
[k256](https://crates.io/crates/k256) in `core::crypto::ecdsa` for
`non-std` environments as outcome of discussion in #3448.

`secp256k1` is used in `std`, meaning that we should not affect host
performance with this PR.
`k256` is enabled in runtimes (`no-std`), and is required to proceed
with #2044.

If desirable, in future we can switch to `k256` also for `std`. That
would require some performance evaluation (e.g. for EVM chains as per
https://github.com/paritytech/polkadot-sdk/issues/3448#issuecomment-1976780391).

Closes https://github.com/paritytech/polkadot-sdk/issues/3448

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Co-authored-by: command-bot <>
Co-authored-by: Davide Galassi <[email protected]>