feat: add managed card merchants feature

[tessstoddard]

Summary of Changes

• https://mxcom.atlassian.net/browse/GCU-584
• https://developer.mx.com/drafts/mdx/managed_cards/#merchants

Adds list/get/update endpoints for managed card merchants

Downstream Consumer Impact

New functionality. Shouldn't affect anyone

How Has This Been Tested?

Did some super basic tests to verify this works in a connector

List merchants:

GET /managed_cards/1/merchants

HTTP/1.1 200

{
  "merchants": [
    {
      "id": "merchant1",
      "logo_url": "logoUrl1",
      "name": "Merchant 1",
      "status": "ALLOWED",
      "user_id": "U-00ue70ev9bAHgQq2A1d7"
    },
    {
      "id": "merchant2",
      "logo_url": "logoUrl2",
      "name": "Merchant 2",
      "status": "BLOCKED",
      "user_id": "U-00ue70ev9bAHgQq2A1d7"
    }
  ]
}

Get merchant:

GET /managed_cards/1/merchants/merchant1

HTTP/1.1 200

{
  "merchant": {
    "id": "merchant1",
    "logo_url": "logoUrl1",
    "name": "Merchant 1",
    "status": "ALLOWED",
    "user_id": "U-00ue70ev9bAHgQq2A1d7"
  }
}

Update merchant:

PUT /managed_cards/1/merchants/merchant1

{
  "merchant": {
    "status": "BLOCKED"
  }
}

HTTP/1.1 200 

{
  "merchant": {
    "id": "merchant1",
    "logo_url": "logoUrl1",
    "name": "Merchant 1",
    "status": "BLOCKED",
    "user_id": "U-00ue70ev9bAHgQq2A1d7"
  }
}

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works

[stevecl5]

LGTM!