Add `generateMultikey` helper to KeystoreAgent to aid in simple generation of multikey formatted keys

It can be used as an alternative to generateKey and should take the key algorithm instead of a generic string type param (which is internally mapped in generateKey). It should look something like this:

async generateMultikey({
  algorithm, capability, maxCapabilityChainLength,
  publicAlias, publicAliasTemplate
} = {}) {
  const {capabilityAgent, kmsClient} = keystoreAgent;
  const invocationSigner = capabilityAgent.getSigner();
  const fullType = `urn:webkms:multikey:${algorithm}`;
  const {keyId, keyDescription} = await kmsClient.generateKey({
    type: fullType,
    // make this a const somewhere
    suiteContextUrl: 'https://w3id.org/security/suites/multikey/v1',
    invocationSigner, capability,
    maxCapabilityChainLength, publicAlias, publicAliasTemplate
  });
  const {id, type} = keyDescription;
  return new AsymmetricKey({
    id, kmsId: keyId, type, invocationSigner, kmsClient, keyDescription
  });
}

Start with the KeystoreAgent's existing generateKey path and the AsymmetricKey construction it uses. Add the generateMultikey entry point with the listed options, multikey type and suite context, and confirm it returns an AsymmetricKey with the generated key details.