Provide FPR to tag plugins.

The module currently provides some limited meta-data API for tag plugins to add attachments via TagPluginInterface::getAttachments().

Both this, and other meta-data stuff (like lazy building, placeholders, caching) could be made much easier by providing plugins with access to the FilterProcessResult.

Or, if that's too powerful, letting tags return their own form of this, a TagProcessResult, which will then be merged together into the FPR.

(This could eventually allow context awareness, as in this: #226963: Context-aware text filters (provide more meta information to the filter system))