Support for integration of Agents-Plugin spec v1

# What are you trying to achieve?

Currently API-Center allows to integrate to a GitHub Repository and check for Agents, Skills and MCPs, but not for any plugins, but the API-Center Portal itself supports showing plugins. So it makes sense from my perspective to support the newest https://agent-plugins.org/ spec as another integration feature.

# Proposed solution

Support regex for searching plugin.json file in a GitHub repository and syncing them in the API Center like Agents, Skills and MCPs. In best case supporting all official metatags, but at least name, description and version.

# Alternative solutions

None, its time to support it :)

# Additional context

See the official https://agent-plugins.org/

Start by reading the existing GitHub repository integration for Agents, Skills, and MCPs, then compare its discovery and syncing flow with the official Agents-Plugin spec. The change is done when plugin.json files are found and synced into API Center with at least their name, description, and version, with official metadata supported where feasible.

Written by the indexing model from the issue text.