Add support for appending attributes to KeyInfo element

[shunkica]

Added a new optional property, attrs, to the KeyInfo interface.
The attrs property is an object that holds key-value pairs representing the attributes and their corresponding values.
When present, these attributes will be appended to the KeyInfo element during XML generation.

Example:

sig.keyInfoProvider = {
    getKeyInfo: () => {
        return `<wsse:SecurityTokenReference xmlns:wsse="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd" xmlns:wsu="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-utility-1.0.xsd" wsu:Id="STR-${securityTokenReferenceGuid}">`
            + `<wsse:Reference URI="#X509-${randomUUID()}" ValueType="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-x509-token-profile-1.0#X509v3"/>`
            + `</wsse:SecurityTokenReference>`
    },
    getKey: () => Buffer.from(privateKey),
    attrs: {
        Id: `KI-${randomUUID()}`,
    }
};

Result:

<ds:KeyInfo Id="KI-a8bb2a1a-32dd-449e-9bf0-187dd8ea4ee6">
  <wsse:SecurityTokenReference wsu:Id="STR-0096ce19-3fbf-414f-94e9-f7a0d601b3f4">
    <wsse:Reference URI="#X509-8cc9b13b-ee21-426e-b8d1-75cddddbe6e5"
      ValueType="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-x509-token-profile-1.0#X509v3"/>
  </wsse:SecurityTokenReference>
</ds:KeyInfo>