What does this PR do?

This PR changes multiple things:

Docker image releases

This PR refactored the way docker images (like service-extensions or request-mirror) are built.
The build and upload workflow is now a reusable component.

The docker images of the repo are now released at the same time as the tracer.

Service Extensions / External Processing in system tests

This PR build a service extension image based on the latest commit (all PRs) and run the EXTERNAL_PROCESSING system tests scenarios on that image.

Changes:

• Move out the EXTERNAL_PROCESSING scenarios out of the system-tests job
• Created a build step for the docker image (using the new refactored workflow component)
• Created a new system-tests job only for the external processing (downloading the built image and run the selected scenarios on it)
• On main, on each new commits, the service extensions docker image used for the system tests is uploaded to the registry with the dev tag (that will be later used by the system-tests repo to run the tests as Dev)

Motivation

• Easier release of the docker images
• We want to run continuously the system tests on the latest version of the service extension image. Currently it was only ran when the image was released (so after a tracer release).

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• New code is free of linting errors. You can check this by running golangci-lint run locally.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

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