Support mTLS for receptive agents

Follow-up issue to #60 (closed).

Add support for mTLS (mutual TLS) for receptive agents.

Ref: gitlab-org/cluster-integration/gitlab-agent!1764 (merged)

Proposal

• Support --api-mtls=true flag
• ⚠ TLS termination at Ingress doesn't work with mTLS. We need to stop TLS termination at Ingress when mTLS is configured.
• Verification steps:
  • Run GDK
  • Enable receptive mode
  • Create URL configuration (curl) with cert & key for mTLS
  • Verify the happy path
  • Verify the invalid path