Add setPostInteractionSettings and related prefs

[estrattonbailey]

Introduces a new preferences object whereby the user can specify their preferred default threadgate/postgate rules. This data will be used to pre-fill their post interaction settings when creating new posts. It will not apply to previously existing posts.

agent.setPostInteractionSettings({
  threadgateAllowRules: AppBskyFeedThreadgate.Record['allow']
  postgateEmbeddingRules: AppBskyFeedPostgate.Record['embeddingRules']
})

I also added some clarifying descriptions to the lexicons so that they output to the types. The handling around these is a little different, and unclear. Main thing: threadgate['allow'] being set to undefined means "no rules specified, anyone can reply" and [] means "no allow rules were added by the user, no one can reply". This is some historical baggage that would require a larger refactor to handle, and so it's not handled here.

[rafaelbsky]

I want to merge #3496 soon. You could include the new rule here before merging this.