Add an upgrade integration test upgrading to the PR build

[AndersonQ]

Describe the enhancement:

The integration test should install and enroll the latest official release Elastic Agent, then upgrade it to the agent built from the PR.

The following is required:

• Sing the agent package built from the PR. (use pgptest.Sing
• Setup a http server serving the ASCII armoured public key used to sign the agent artifact.
  • the agent downloads it from https://artifacts.elastic.co/GPG-KEY-elastic-agent.
  • See PGP fallback URL in air-gapped environment workaround for details.
• setup a http server to serve the agent binary, its signature and checksum file for the upgrade
• configure the policy to use the custom agent download binary URL

Describe a specific use case for the enhancement or feature:

To test the migration of the state store during upgrade

What is the definition of done?

The test is created and successfully runs together with the current integration tests

[AndersonQ]

@pierrehilbert, @cmacknz as we discussed in the weekly, I created this issue for the new upgrade test

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)