Add basic http respond to respond to Azure pings

[Pactionly]

Azure App Service requires containers to respond to http pings to be consider healthy. Configuring Gratibot to use socket mode disabled its ability to respond to these pings and caused the service to be flagged as unhealthy.

Ideally we'd configure Gratibot with custom routes as part of the receiver package in Bolt, however, the SocketModeReceiver doesn't currently support custom routes. See: slackapi/bolt-js#834

Until support is added (Milestone 3.5.0), this PR adds a basic express response to satisfy Azure's requirements.

[github-actions]

Terraform Nonprod Plan success

Show Plan

An execution plan has been generated and is shown below.
Resource actions are indicated with the following symbols:
  ~ update in-place

Terraform will perform the following actions:

  # azurerm_app_service.gratibot_app_service will be updated in-place
  ~ resource "azurerm_app_service" "gratibot_app_service" {
      ~ app_settings                      = {
          - "DOCKER_REGISTRY_SERVER_URL"  = "https://index.docker.io/v1" -> null
            # (5 unchanged elements hidden)
        }
        id                                = "/subscriptions/761bf739-12c3-43cb-bec9-d87c1d7fa459/resourceGroups/gratibot-azure-data/providers/Microsoft.Web/sites/gratibot-nonprod-service"
        name                              = "gratibot-nonprod-service"
        tags                              = {}
        # (14 unchanged attributes hidden)




      ~ site_config {
          ~ linux_fx_version            = "DOCKER|liatrioalicej/gratibot:3.0.0" -> "DOCKER|ghcr.io/liatrio/gratibot:0830aa4"
            # (17 unchanged attributes hidden)

            # (1 unchanged block hidden)
        }

        # (4 unchanged blocks hidden)
    }

Plan: 0 to add, 1 to change, 0 to destroy.