allow configuration of the security context of the security agent

[paulcacheux]

What this PR does / why we need it:

This PR syncs the configuration of the security context of the security agent with how it's done for other agents (especially system probe).

The end motivation is to give access to other fields of the security context like allowPrivilegeEscalation which was not currently possible.

Which issue this PR fixes

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

• fixes #

Special notes for your reviewer:

Checklist

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

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