Add support for Github Packages

[1aziz]

Adding a Docker login step into the pipeline in order to push images to the Github Packages instead of DockerHub.

See pushed images: https://github.com/orgs/ContainerSolutions/packages/container/package/externalsecret-operator

[codecov[bot]]

Codecov Report

Merging #147 (987d22b) into master (89d57f4) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #147   +/-   ##
=======================================
  Coverage   70.55%   70.55%           
=======================================
  Files          10       10           
  Lines         411      411           
=======================================
  Hits          290      290           
  Misses        104      104           
  Partials       17       17           

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[1aziz]

Work will be ready to be merged after:

• Adding the missing (new) environment variables. I'm not authorized yet to add secrets into the workflow.
• Temporarily modifying pipeline triggers (for testing)

[knelasevero]

/approve