Support feature flags

[valner]

No description provided.

[Copilot]

Pull request overview

Adds end-to-end support for server-provided feature flags by persisting them into a systemd EnvironmentFile and triggering agent restarts when flags change, plus a Docker-based integration test harness to validate the behavior.

Changes:

• Implement feature-flag validation, environment file generation, and restart gating logic in the updater application.
• Add OS/systemd helper support for restarting services and extend the agent interface to expose an environment file path.
• Introduce Docker Compose–based integration tests with a mock gRPC server and a fake agent, and wire these into CI.

Reviewed changes

Copilot reviewed 18 out of 19 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
internal/application/application.go	Adds feature flag validation, env file writing, and restart decision logic.
internal/application/application_test.go	Adds unit tests for env file generation/validation and restart gating behaviors.
internal/agents/interface.go	Extends AgentData with GetEnvironmentFilePath().
internal/agents/o11y-agent.go	Implements env file path + real restart via systemd for O11y agent.
internal/osutils/oshelper.go	Adds RestartService() helper used by agent restart.
internal/client/client.go	Supports insecure gRPC credentials when configured.
internal/client/client_test.go	Updates mocks to satisfy the extended agent interface.
tests/integration/* (multiple files)	Adds mock server, fake agent, docker images/compose, and integration test suite covering feature flags + restarts.
Makefile	Separates unit tests from integration tests and adds integration-test target.
.github/workflows/check.yaml	Runs integration tests in CI.
go.mod / go.sum	Adds Docker client dependencies and updates module versions required by new tests.

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