#2719 Create GH Action for building the CLI

[christian-kreuzberger-dtx]

This PR introduces a GitHub Action/Workflow that builds the CLI and uploads it as an artifact to the action itself (not to gcloud for now).

Note: The release step is not fully implemented yet. CLI is currently uploaded to GH actions instead of google cloud, but I guess that's a good thing.

In addition, I've removed all scripts that were used to build the CLI via travis-ci previously.

New Features

• Build CLI on the operating system it is meant for (windows, linux, macos)
• Upload CLI build artifacts via GH actions

GH Actions

Looks as follows:

Build CLI on {Linux, Windows, MacOS}

This builds the cli on the target platform, and uploads an artifact to GitHub.

Release Keptn

Not implemented yet, but here is the idea

This is only triggered for release branches (e.g., release-0.8.0).
It downloads the previously built artifact, creates a draft release, and uploads the CLI assets to the draft release.

The end result of this step is a draft release with up2date CLI binaries (packed as .tar.gz):

[codecov-io]

Codecov Report

Merging #2795 (8ef9831) into master (09faf22) will decrease coverage by 0.38%.
The diff coverage is 25.00%.

@@            Coverage Diff             @@
##           master    #2795      +/-   ##
==========================================
- Coverage   36.24%   35.85%   -0.39%     
==========================================
  Files         161      158       -3     
  Lines        9370     9189     -181     
  Branches      197      197              
==========================================
- Hits         3396     3295     -101     
+ Misses       5430     5377      -53     
+ Partials      544      517      -27     

Flag	Coverage Δ
moduleA	22.61% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
api/handlers/evaluation.go	0.00% <0.00%> (ø)
cli/cmd/root.go	19.29% <ø> (-2.74%)	⬇️
helm-service/controller/action_handler.go	85.71% <ø> (-0.21%)	⬇️
helm-service/controller/delete_handler.go	87.80% <ø> (-0.30%)	⬇️
helm-service/controller/deployment_handler.go	33.65% <ø> (-0.64%)	⬇️
helm-service/controller/onboarder.go	89.85% <ø> (-0.08%)	⬇️
helm-service/controller/release_handler.go	22.32% <ø> (-0.69%)	⬇️
mongodb-datastore/handlers/events.go	23.38% <0.00%> (ø)
cli/cmd/configure_monitoring.go	41.89% <8.00%> (-15.81%)	⬇️
cli/pkg/credentialmanager/handler.go	13.04% <28.00%> (+2.69%)	⬆️
... and 5 more

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[christian-kreuzberger-dtx]

Building on MacOS ic currently not working due to a Datacenter upgrade: actions/runner-images#2247
Though we can still merge this.

[christian-kreuzberger-dtx]

No longer needed, superseeded by #2817