`indexable` and `discoverable` attributes for users and communities

Requirements

• Is this a feature request? For questions or discussions use https://lemmy.ml/c/lemmy_support or the matrix chat.
• Did you check to see if this issue already exists?
• Is this only a feature request? Do not put multiple feature requests in one issue.
• Is this a backend issue? Use the lemmy-ui repo for UI / frontend issues.
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Is your proposal related to a problem?

At the moment, neither Lemmy communities nor users (nor their posts/comments) can be searched for on platforms such as Mastodon

Describe the solution you'd like.

At the moment, you can search for posts, communities, and users on e.g. other Lemmy instances, but not from (for example) Mastodon, because the actors lack the toot:indexable and toot:discoverable attributes set to true. Adding these attributes to both users and communities would allow them to be searched for more easily on platforms that implement said attributes, and would allow for them to appear in search results on services such as Holos Discover.

Describe alternatives you've considered.

N/A

Additional context

No response