Add pipeline templates

[premun]

Adds the possibility to add template: file.yml with parameters to any part of the pipeline.

Example:

Stages =
{
    new Stage("Build_Windows_NT", "Build Windows")
    {
        Jobs =
        {
            new Template<Job>("/eng/common/templates/jobs/jobs.yml")
            {
                Parameters =
                {
                    { "enableTelemetry", true },
                    { "helixRepo", "dotnet/xharness" },
                    { "SDLValidationParameters", new TemplateParameters
                        {
                            { "enable", false },
                            { "continueOnError", false },
                        }
                    }
                }
            }
        }
    },
}

which produces this YAML:

stages:
- name: Build_Windows_NT
  displayName: Build Windows
  dependsOn: []
  variables: []
  jobs:
  - template: /eng/common/templates/jobs/jobs.yml
    parameters:
      enableTelemetry: true
      helixRepo: dotnet/xharness
      SDLValidationParameters:
        enable: false
        continueOnError: false