feature: add extra_envs variable to agent module

[agustincelentano]

Summary

• Adds extra_envs variable (map(string)) to the agent module
• Allows passing additional environment variables to the agent without modifying module internals
• Merges extra envs into all_config, so they get injected via the existing Helm values template

Usage

module "agent" {
  source = "..."
  # ...
  extra_envs = {
    MY_CUSTOM_VAR = "value"
  }
}

[sebastiancorrea81]

lgtm