discovery: add useSdAgent config option

[Yumasi]

What this PR does / why we need it:

This PR adds a new datadog.discovery.useSdAgent config option, that sets the corresponding system-probe configuration option. When this option is enabled, system-probe is wrapped by the new sd-agent binary, which will run when only discovery is enabled. If any other system-probe feature is required, sd-agent automatically fallbacks to system-probe.

The PR also changes the command line of the system-probe container to first check if the sd-agent binary exists first. This is needed to support older agent versions. This was tested with a deployment of 7.75 with the updated chart, and useSdAgent set to true.

Which issue this PR fixes

(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged)

• fixes DSCVR-271

Special notes for your reviewer:

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• All commits are signed (see: signing commits)
• Chart Version semver bump label has been added (use <chartName>/minor-version, <chartName>/patch-version, or <chartName>/no-version-bump)
• For datadog or datadog-operator chart or value changes, update the test baselines (run: make update-test-baselines)

GitHub CI takes care of the below, but are still required:

• Documentation has been updated with helm-docs (run: .github/helm-docs.sh)
• CHANGELOG.md has been updated
• Variables are documented in the README.md