feat(merchant_account): Add list multiple merchants in `MerchantAccountInterface` (#3220)

Co-authored-by: hyperswitch-bot[bot] <148525504+hyperswitch-bot[bot]@users.noreply.github.com>
Files to modify:


**merchant_account.rs**
  Add:
    - function: pub::list_multiple_merchant_accounts

**merchant_key_store.rs**
  Add:
    - function: pub::list_multiple_key_stores

**kafka_store.rs**
  Add:
    - function: private::list_multiple_merchant_accounts
    - function: private::list_multiple_key_stores

**merchant_account.rs**
  Add:
    - function: pub::list_multiple_merchant_accounts

**merchant_key_store.rs**
  Add:
    - function: pub::list_multiple_key_stores