feat: add support for log format configuration

[fad3t]

What this PR does / why we need it:

Sets the DD_LOG_FORMAT_JSON env variable to make the agents log messages in the JSON format. This is controlled with the new variable datadog.logFormat (options are text (default) and json).

Checklist

• [X] Chart Version bumped
• [X] Documentation has been updated with helm-docs (run: .github/helm-docs.sh)
• [X] CHANGELOG.md has been updated
• [X] Variables are documented in the README.md
• [ ] For Datadog Operator chart or value changes update the test baselines (run: make update-test-baselines)

[mathieux51]

@fad3t Exactly the feature I was looking for 😁. Hope it gets merged soon 👀!