[build] Add build-tools/automation/build.groovy (#2603)

Context: http://groovy-lang.org/syntax.html
Context: https://jenkins.io/doc/book/pipeline/syntax/#scripted-pipeline

Historically, the [`xamarin-android` Jenkins build][0] has been a
[Jenkins Freestyle project][1].  While this works, there are a few
problems we've experienced with Freestyle projects:

  * The Freestyle configuration is not publicly visible, meaning
    external contributors can't verify their own environment when
    they encounter local build failures.

  * The Freestyle configuration is revision controlled, but the
    revisions do not have commit messages, and are not associated
    with the `xamarin-android` repo.

  * There Can Be Only One **Upload artifacts to Microsoft Azure
    Storage** step.

We would like to have a world in which we build the `xamarin-android`
repo, create the installers, upload the installers, build and run the
unit tests, and then upload the results of the unit test execution.
This requires (at least) two separate upload steps.

The final bullet point means we cannot do this with Freestyle
projects, which is doubly annoying when we encounter build timeouts,
as the mono bundle and OS installers will exist but won't be uploaded
and available for further use until after the *entire* build is
complete, which could be a significant amount of time.

Instead of using a Jenkins Freestyle project, we will migrate to a
[Jenkins Scripted Pipeline build][2].  Jenkin scripted pipeline
builds involve a [Groovy][3] script which will be executed to perform
the build within Jenkins, fixing all of of the Freestyle project
limitations detailed above:

  * The Groovy script will be stored within the `xamarin-android`
    repo, and will be publicly visible.

  * As the Groovy script will be kept within the `xamarin-android`
    repo, it can undergo normal PR review and have useful commit
    messages describing the changes.

  * Scripted pipeline builds can upload multiple times.  This will
    allow us to upload installers immediately after they're created,
    which will allow "downstream workflows" to be kicked off without
    them needing to wait for the entire build to be completed.

Add a new `build-tools/automation/build.groovy` file, which is the
Groovy script to use for the Jenkins scripted pipeline build.

`build.groovy` requires that the build environment provide the
following values:

  * `BotLabel`: Label used for bot selection on which machine the
    build will run (e.g. `xamarin-android-master`)
  * `BuildFlavor`: Target build flavor for the build such as
    `Release` or `Debug`
  * `ContainerName`: The name of the Azure container where the
    artifacts will be stored
  * `StorageAccountName`: Name of Azure storage account using to
    store published artifacts
  * `StorageCredentialId`: Credential id for Azure storage account
    used to store the published artifacts
  * `StorageVirtualPath`: The name of the Azure path (folder) where
    published artifacts will be stored

[0]: https://jenkins.mono-project.com/view/Xamarin.Android/job/xamarin-android/
[1]: https://www.guru99.com/create-builds-jenkins-freestyle-project.html
[2]: https://jenkins.io/doc/book/pipeline/syntax/#scripted-pipeline
[3]: http://groovy-lang.org/syntax.html

Author: mjbond-msft