Make associated_inputs metric extensible to plugin input types

[AntonEbel]

Description

associated_inputs on streams only knew about the open-source inputs collection, so the IDs it returned could only resolve to regular input titles and were filtered with a single hard-coded inputs:read check. Plugin-contributed input kinds (e.g. forwarder inputs) had no way to plug their own resolution or permission check into the metric.

Approach

• Introduce a pluggable InputType SPI for declaring an input kind's read permission and id-existence check. Each id is tagged with its type at compute time so per-user filtering on read is a pure Shiro check.
• Add frontend plugin slots so each input type can supply its own title/detail fetcher and link builder; the stream UI merges them with the regular-input catalog.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

/prd Graylog2/graylog-plugin-enterprise#14472
/nocl

[kodjo-anipah]

Just one nit, LGTM bit awkward probably the best we can do right now without big refactoring