[Omnibus] Support packaging of the agent at a different location than /opt/datadog-agent

[Pythyu]

What does this PR do?

Allow omnibus to package and build the agent at a different location than /opt/datadog-agent. Also gives the option to generate config files at a custom path instead of /etc/datadog-agent.

Note: The packaged agent is still meant to work from /opt/datadog-agent and will still try to get config files from /etc/datadog-agent

Motivation

MacOS runners don't support virtualization and have already a datadog-agent present for internal tracking. We need to be able to build the agent on runners without overwriting on the existing agent.

Additional Notes

This PR depends on this omnibus-ruby PR

Currently tested only on the base flavor Once the macOS builds uses this PR, we can remove this test job since regression will be handled by the macOS build job.

Possible Drawbacks / Trade-offs

Describe how to test/QA your changes

Manual testing

export INSTALL_DIR="/some/custom/path" # needs to be exported
inv omnibus.build --install-directory "$INSTALL_DIR" --config-directory "/some/custom/config/path"

CI Currently custom_agent_build_test create an x64 build to /custom/datadog-agent with config files in /custom/etc/datadog-agent the job fails if not correctly generated at these locations.