docs: expand USAGE.md with complete API reference for all GMP/OSP/HTTP methods

Covers all 200+ GMP methods across every category (tasks, targets,
scanners, scan configs, alerts, credentials, reports, results, audits,
policies, schedules, filters, tags, groups, roles, users, permissions,
notes, overrides, tickets, TLS certs, vulnerabilities, report formats,
report configs, hosts, OS, NVTs, CVEs/CPEs/CERT, port lists, feeds,
aggregates, GMPNext agent/OCI features), OSP protocol, HTTP/OpenVASD
API, XML utilities, all connection types, transforms, and error classes.

https://claude.ai/code/session_01D8fVeN4YVfAbzr3cXG4ZVA

Author: claude