ci: automated release using github actions

[jackfrancis]

Reason for Change:

This PR introduces a new release action to make aks-engine releases automated.

The process, in brief:

1. create a new release branch
2. add a releases/CHANGELOG-.md file with release notes
3. push a new tag that matches the version to be released (e.g., v0.64.0)

Upon pushing the tag, the new github action will run various E2E against that commit, and then conditionally create a new release w/ notes from the versioned CHANGELOG file.

Issue Fixed:

Credit Where Due:

Does this change contain code from or inspired by another project?

• No
• Yes

If "Yes," did you notify that project's maintainers and provide attribution?

• No
• Yes

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests
• tested upgrade from previous version

Notes:

[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jackfrancis

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [jackfrancis]

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[jackfrancis]

cc @jadarsie @Michael-Sinz

[codecov]

Codecov Report

Merging #4451 (5c8d0b8) into master (60828c7) will increase coverage by 0.00%.
The diff coverage is 72.07%.

❗ Current head 5c8d0b8 differs from pull request most recent head e4c634c. Consider uploading reports for the commit e4c634c to get more accurate results

@@           Coverage Diff            @@
##           master    #4451    +/-   ##
========================================
  Coverage   72.04%   72.05%            
========================================
  Files         141      141            
  Lines       21631    21764   +133     
========================================
+ Hits        15584    15681    +97     
- Misses       5096     5131    +35     
- Partials      951      952     +1     

Impacted Files	Coverage Δ
cmd/rotate_certs.go	11.03% <0.00%> (ø)
cmd/upgrade.go	35.92% <0.00%> (ø)
pkg/api/common/versions.go	96.37% <ø> (ø)
pkg/api/types.go	92.85% <ø> (ø)
pkg/api/vlabs/types.go	73.04% <ø> (ø)
pkg/engine/templates_generated.go	43.31% <ø> (ø)
pkg/engine/template_generator.go	66.37% <11.53%> (-2.17%)	⬇️
cmd/get_logs.go	49.57% <30.43%> (-1.79%)	⬇️
cmd/addpool.go	17.97% <100.00%> (ø)
pkg/api/addons.go	98.01% <100.00%> (ø)
... and 5 more

---

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