License Checker for vaadin cval products (#13696 #13474)

- This patch includes four elements:
  1.- A class able to validate a licensed product against Vaadin
      license server. It can be used in any vaadin product (thought
      for non addons like TB) just adding vaadin dependency, or
      copying the class.
  2.- A class able to inspect all addons in the classpath and figure
      out, based on the MANIFEST.MF info, whether we have to check
      developer license.
  3.- A modification to Vaadin connector generator to use the classes
      above and to stop compilation in case.
  4.- A modification to ConnectorBundleLoader, so as when a new connector
      is instatiated, we check whether it is using an evaluation
      license and show a notice. We only show the notice once.

- In addition to validating developer licenses, the checker caches the
  server response for using it when there are connection problems.
- This stuff is in Vaadin core, so as we dont maintain license code in
  each addon. For checking an addon license we just add the license type
  to the manifest when packaging the artefact.
- It checks expiration time, product name and major version.

Fixes: #13696
In some-way related with: #13474

Change-Id: Ib61b1c2e9c3cacd463a1ce5db02c2cfbc06851c2