Add Xcode version selection option (Xcode 10.2 now default on Firebase Test Lab)

[nancy-gomez]

Hello! So I am in the Firebase Community Slack group and I saw in the #ftl-announcements room there was an announcement:

""" Hey everyone! For iOS, it's looking like we'll roll out Xcode 10.2 next week [Stated on Friday April 19th]. As a reminder, if you're still building with Xcode 10.1, please specify it as an argument to either gcloud or Flank using --xcode-version=10.1, as the new default will immediately be 10.2 (and it's looking like not all tests are forwards-compatible).

Xcode 10.2 is live! Note that for those using the default Xcode version, you'll switch over automatically to Xcode 10.2. """

And was wondering if we can add an additional option flag for specifying which xcode version to use :)

[MartyCatawiki]

Hi @nancy-gomez, for me the latest Xcode version works perfect, but i can imagine that you would like to specify the Xcode version sometimes. Please feel free to make a PR for this and then the code-owners can review, you can see the API possibilities here: https://testing.googleapis.com/$discovery/rest?version=v1

[LukeDurrant]

See https://github.com/fastlane/fastlane-plugin-firebase_test_lab/pull/65

[nancy-gomez]

@LukeDurrant Oh my gosh thank you! I was barely going to set aside some time next week to try and do this myself. :D Thank you!

[nancy-gomez]

Hey @LukeDurrant why did you close your PR? Do you plan to make another PR?