Nightly and build test for XPK

[sushmarchandran]

Fixes / Features

• Integrated Github Actions with TPUs on gcloud to run build and nightly tests
• Fixed #23
• Running nightly tests and build tests on push and PR approval
• Used Service Accounts to authenticate gcloud on Github Actions
• Created a CCA bug to request reservation capacity for tests in the future

Testing / Documentation

• Documented xpk/.github/workflows/README.md to reflect the changes. Linked xpk/README.md to point to this document.
• Tests added to create and delete: 1. A cluster with 0 nodepools, and, 2. a cluster with 2 slices and v4-8 TPU nodes
• [ y ] Tests pass
• [ y ] Appropriate changes to documentation are included in the PR

[sushmarchandran]

As discussed offline with @Obliviour, the following changes are requested:

• remove empty cluster creation for build tests. Build tests will only run 2 slice v4-8 cluster creation and deletion
• No tests will run for PR creation. 2 slice v4-8 cluster creation and deetion tests are run on PR approval and push to main