feat: add proper enabled config for plugins

[Noroth]

Motivation and Context

We recently removed the enabled flag for the plugins as it didn't make sense the way it was implemented. Instead of enabling only the configuration options this PR adds an implementation to handle enabling of plugins in general.

Enabled now will make sure that no plugins are loaded when enabled is set to false.

Checklist

• I have discussed my proposed changes in an issue and have received approval to proceed.
• I have followed the coding standards of the project.
• Tests or benchmarks have been added or updated.
• Documentation has been updated on https://github.com/wundergraph/cosmo-docs.
• I have read the Contributors Guide.

[github-actions]

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✅ No security vulnerabilities found in image:

ghcr.io/wundergraph/cosmo/router:sha-3e1c8ddb3af7b5f9f630fb2e65711dd53542e2b9