feat(build-push-to-gar): Expose platforms parameter

[andriikushch]

This PR is inspired by:

• https://github.com/grafana/shared-workflows/tree/main/actions/build-push-to-dockerhub

It exposes the platforms parameter and will allow to make builds for multiple platforms/architectures.

[CLAassistant]

All committers have signed the CLA.

[iainlane]

could you add this to the inputs block at the top too?

[andriikushch]

could you add this to the inputs block at the top too?

Good point @iainlane 👍 I have missed it. Now it should be there 😃

[iainlane]

thanks!