[proposal] Add temp memory key generation function

[nklomp]

Is your feature request related to a problem? Please describe.
For the DID ION keys in pr #987 I needed to pre-create keys for rotation and recovery. I also needed to be able to assign specific key ids (kid). As I didn't want to duplicate all the logic to retrieve the public keys from different key types, I ended up basically using an in memory KMS for temporary keys (see below). It might be helpful to others to have this exposed as a function

Describe the solution you'd like
expose a function that allows IKey generation completely in memory

Additional context
Right now the DID Ion Provider is dependant on kms-local. If Veramo would provide this function that coupling could be removed.

The below line is the import one.

  const tmpKey = (await new KeyManagementSystem(new MemoryPrivateKeyStore()).importKey({
    type,
    privateKeyHex,
    kid,
  })) as IKey

The full reference how I used it in the ION provider

/**
 * Create a Veramo Key entirely in Memory. It is not stored
 *
 * Didn't want to recreate the logic to extract the pub key for the different key types
 * So let's create a temp in-mem kms to do it for us
 *
 * @param type
 * @param privateKeyHex
 * @param kid
 * @param kms
 * @param ionMeta
 */
export const tempMemoryKey = async (
  type: KeyType.Ed25519 | KeyType.Secp256k1 | KeyType,
  privateKeyHex: string,
  kid: string,
  kms: string,
  ionMeta: IonKeyMetadata
): Promise<IKey> => {
  const tmpKey = (await new KeyManagementSystem(new MemoryPrivateKeyStore()).importKey({
    type,
    privateKeyHex,
    kid,
  })) as IKey
  tmpKey.meta!.ion = JSON.parse(JSON.stringify(ionMeta))
  tmpKey.meta!.ion.commitment = computeCommitmentFromJwk(toIonPublicKeyJwk(tmpKey.publicKeyHex))
  tmpKey.kms = kms
  // tmpKey.privateKeyHex = privateKeyHex
  return tmpKey
}

[stale]

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