add health module

[KARTIKrocks]

Added

• health — New health package for health check endpoints with dependency checking, timeouts, and standard response formats
• health — NewChecker(opts...) with functional options pattern and configurable per-check timeout (default 5s)
• health — AddCheck(name, fn) registers critical checks — failure sets status to "unhealthy" (503)
• health — AddNonCriticalCheck(name, fn) registers non-critical checks — failure sets status to "degraded" (200)
• health — Check(ctx) runs all checks concurrently with per-check timeout and returns aggregated Response
• health — Handler() returns an error-returning HTTP handler for readiness probes (compatible with router package)
• health — LiveHandler() returns a liveness probe handler that always responds 200