[AWX executor plugin] Add AWXApiClient auth and launch support

[weiyentan]

Parent: #10
Estimate: ~25 min

Implement the minimal AWX API client path needed to authenticate and launch a job template.

Acceptance criteria

• Add AWXApiClient with configurable AWX base URL and token
• Send authenticated requests to the AWX API
• Implement launch_job_template(template_id, extra_vars)
• Return a typed response containing at least the AWX job ID and launch status
• Translate HTTP/connection failures into clear AWX client exceptions

Dependencies

• Blocked by: [AWX executor plugin] Map AWX templates, credentials, and launch contract #79

Parent issue: #10