aibridge: expose certificate via API endpoint

[dannykopping]

RFC

Acceptance criteria:

• GET /api/v2/aibridge/proxy.pem returns the certificate specified in CODER_AIPROXY_CERT_FILE
• The endpoint is for authenticated use only
• Must 404 if CODER_AIPROXY_ENABLED=false or the cert is not defined
• Has tests
• Must NOT expose CA private key