Updates the interface for implementing summarization strategies and adds a cache layer to summarize topics once.
The cache stores the final summary and each chunk used to build it, which will be useful when we have to extend or rebuild it.
* FEATURE: Content custom summarization strategies.
This PR establishes a pattern for plugins to register alternative ways of summarizing content by extending a class that defines an interface.
Core controls which strategy we'll use and who has access to it through the `summarization_strategy` and `custom_summarization_allowed_groups`. It also defines the UI for summarizing topics.
Other plugins can access this summarization mechanism and implement their features, removing cross-plugin customizations, as it currently happens between chat and the discourse-ai plugin.
* Group membership validation and rate limiting
* Work with objects instead of classes
* Port summarization feature from discourse-ai to chat
* Rename available summaries to 'Top Replies' and 'Summary'
