Smarter pick key for credential management

[jasny]

When updating a credential status and keyRef is not specified, the first key of the identifier is picked to sign the transaction. This is allowed according to the LTO specs. However, it would be nicer to use the same key that was used to sign the credential by default.

lto-veramo-plugin/src/lto-credential-plugin.ts

Lines 212 to 213 in fa2a764

	const identifier = await this.getIdentifier(vc.issuer, context?.agent);
	key = this.pickSigningKey(identifier, options?.keyRef);

If the preferred key is no longer available, use the first key.