Allow substrate network to be specified on key generation

[ansermino]

This PR exposes the ability to specify a network ID when generating keys with GSRPC. We should extend the accounts generate command to allow a user to specify this when generating sr25519 (substrate) keys.

Here's where the key generation gets called by the CLI:

ChainBridge/cmd/chainbridge/account.go

Lines 354 to 359 in a92ea6f

	if keytype == crypto.Sr25519Type {
	// generate sr25519 keys
	kp, err = sr25519.GenerateKeypair()
	if err != nil {
	return "", fmt.Errorf("could not generate sr25519 keypair: %s", err)
	}

We'll need to update this method to pass the network string:

ChainBridge/crypto/sr25519/sr25519.go

Line 21 in a92ea6f

func GenerateKeypair() (*Keypair, error) {

(Probably best to use a local replace directive if the PR is still pending, issues do a replace with branch name golang/go#30851 (comment))