feat(collectors): support collector_author override on collector registration

[SamuelHassine]

Use case

OpenAEV attributes a collector's payloads (and their arsenal contracts) to an author
organization. The platform derives it from the collector's name by default, which is a
good author in most cases ("CrowdStrike Endpoint Security", "Atomic Red Team", ...).

However, sometimes the collector name is not a good author (e.g. a deployment-specific
or descriptive name). The platform now supports an optional collector_author field on
collector registration (POST /api/collectors) that overrides the name-based fallback,
mirroring the existing injector_author field on injector registration.

Proposed change

Add collector_author support to pyoaev:

• CollectorManager._create_attrs: declare the optional collector_author attribute.
• CollectorDaemon._setup(): forward the optional collector_author configuration key
  in the registration payload (None when not configured, which keeps the platform's
  name-based fallback).