Add nesting client to support VM-isolated build environments

What does this MR do?

Why was this MR needed?

What's the best way to test this MR?

concurrent = 4
check_interval = 0

[session_server]
  session_timeout = 1800

[[runners]]
  name = "macrunner"
  url = "https://gitlab.com"
  token = "<snip>"
  executor = "instance"

  [runners.instance]
    allowed_images = ["*"]

  [runners.autoscaler]
    capacity_per_instance = 2
    max_use_count = 0
    max_instances = 2
    plugin = "fleeting-plugin-aws"

    [[runners.autoscaler.policy]]
      idle_count = 2
      idle_time  = "24h"

    [runners.autoscaler.connector_config]
      username = "ec2-user"
      key_path = "macos-key.pem"
      timeout  = "1h"

    [runners.autoscaler.plugin_config]
      name   = "mac2metal"
      region = "us-west-2"

    [runners.autoscaler.nesting]
      enabled      = true
      nesting_host = "unix:///Users/ec2-user/Library/Application Support/nesting.sock"
      
    [runners.autoscaler.nesting.connector_config]
      username = "admin"
      password = "admin"
      timeout  = "1h"

What are the relevant issue numbers?

Closes #29345 (closed)