Setup the Release pipeline

[nkammah]

Description

Migrate the Jenkins Release pipeline to Buildkite.

Notes:

• I moved all the logic to load the GPG keys into the pre-command hook to benefit from secrets redaction features and move away from the file based approach used in the .ci/Makefile
• With the above point, I had to rename an env. variable to GPG_FINGERPRINT_SECRET instead of GPG_FINGERPRINT so that Buildkite would always redact it and we don't risk leaking it (with a set -x or printenv). I don't think it would affect anyone since this is used only in the CI flow.
• The k8s agent used for the build is rather big (`cpu: "8" memory:"32G") but it seems needed as go-releaser is very hungry on cross-compiling all the targets in parallel - I had to iterate quite a bit to get to that combo.
• Related to the above, I re-ordered the goarch options to put amd64 at the end which helped with getting the build to succeed as opposed to getting to resource exhaustion - i'm not too sure why.
• The .ci/Makefile should still work to run the flow as a developer instead of ci.

Related Issues

Motivation and Context

How Has This Been Tested?

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (improves code quality but has no user-facing effect)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation

Readiness Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

[dimuon]

LGTM

[jamietanna]

Is this the right issue number? Add config for number of parallel builds #304?