Migrate CI from AWS queues to Google Kubernetes Engine queues

[mstifflin]

What changed?

• Update Buildkite pipeline yaml to work with the newly provisioned queues in Google Kubernetes Engine
• Use agent-stack-k8s v0.8.0 helm chart which has its own expected pipeline yaml syntax in order to successfully onboard.
• Install buildkite-agent in Dockerfile for use in the code coverage step
  • The mount that previously worked in AWS's VM based infra doesn't work in Kubernete's container based set up. Install the cli directly for simplicity.

Why?

• Migrate from AWS > Google Cloud. Buildkite Enterprise recommends GKE (as opposed to GCP) as the way to have a queue with autoscaling compute.

How did you test it?

• https://buildkite.com/uberopensource/cadence-go-client/builds/2175

Potential risks

• CI builds will be broken or flaky
• Can be mitigated by a git revert

[CLAassistant]

All committers have signed the CLA.

[codecov[bot]]

Codecov Report

Merging #1333 (33dd4b5) into master (3ca6328) will increase coverage by 0.06%. The diff coverage is n/a.

Additional details and impacted files

[see 1 file with indirect coverage changes](https://app.codecov.io/gh/uber-go/cadence-client/pull/1333/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=uber-go) ------ [Continue to review full report in Codecov by Sentry](https://app.codecov.io/gh/uber-go/cadence-client/pull/1333?dropdown=coverage&src=pr&el=continue&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=uber-go). > **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=uber-go) > `Δ = absolute (impact)`, `ø = not affected`, `? = missing data` > Powered by [Codecov](https://app.codecov.io/gh/uber-go/cadence-client/pull/1333?dropdown=coverage&src=pr&el=footer&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=uber-go). Last update [3ca6328...33dd4b5](https://app.codecov.io/gh/uber-go/cadence-client/pull/1333?dropdown=coverage&src=pr&el=lastupdated&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=uber-go). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=uber-go).

[mstifflin]

Not sure why some checks don't appear to have reported status, but this PR from 2 weeks ago is similarly impacted: https://github.com/uber-go/cadence-client/pull/1330

But this PR from a month ago is not: https://github.com/uber-go/cadence-client/pull/1324

Please do let me know if I'm missing something here.