Fixes #39290 - Add :registration named logger with session markers

[pablomh]

Summary

Adds a dedicated :registration named logger for host registration events, independently configurable from the main Rails logger.

Changes

• config/application.rb: Register :registration as a named logger
• registration_controller.rb: Emit session_start on GET /register and session_complete on POST /register with host name, action (created/updated), and wall-clock duration
• Test: Verify log lines are emitted with correct content

Log output

# GET /register (script delivery)
[registration] session_start org=Default loc=Default hostgroup=RHEL9

# POST /register (host record creation)
[registration] session_complete host=server.example.com action=created duration_ms=234

Why a dedicated logger

The :registration named logger lets operators enable debug level for registration events without flooding the log with SQL queries or view rendering:

:logging:
  :loggers:
    :registration:
      :level: debug

At debug level, companion Katello PRs emit Candlepin per-call timing and cache HIT/MISS decisions using the same logger.

How to test

1. bundle exec rake test TEST=test/controllers/api/v2/registration_controller_test.rb
2. Register a host and check production.log for session_start and session_complete lines