Maintenance/gradle builds v3

[devzbysiu]

Add gradle builds

Description

We are going to modernize CI/CD and local development. The first step is to move to gradle. This PR adds gradle builds leaving maven still in place.

Motivation and Context

Using gradle allows to implement more flexible build scripts. Thanks to that we can make local development of AETs easier.

Screenshots (if appropriate)

N/A

Upgrade notes (if appropriate)

No changes required. After this PR is merged, you can do ./gradlew or ./gradlew :zip:make in root dir to create AETs artifacts.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• I have reviewed (and updated if needed) the documentation regarding this change

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I hereby agree to the terms of the AET Contributor License Agreement.