Suggestion: Support HEX prefix on private keys

[sondreb]

The .from method on Secp256k1KeyPair takes an option argument that can be privateKeyHex. If this provided value is prefixed with '0x' then the method fails without any errors and simply generates a key pair without the private key.

The change could be applied here (and probably other places where the keys as HEX is allowed):

did-key.js/packages/secp256k1/src/keyUtils.ts

Line 204 in c1bef56

export const privateKeyBase58FromPrivateKeyHex = (privateKeyHex: string) => {

Could be handled with something like this:

privateKeyHex = privateKeyHex.startsWith('0x') ? privateKeyHex.substring(2) : privateKeyHex;