feat(GuildForumThreadManager): add pinned getter

[SSardorf]

Please describe the changes this PR makes and why it should be merged:
Given that there can only be one pinned thread per guild forum, a helper getter is added to provide an easier way to retrieve it.

Status and versioning classification:

• This PR changes the library's interface (methods or parameters added)

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[Jiralite]

Pinned posts are decided with a channel flag—it would be very easy for Discord to say "Hey, let's make the maximum pinned posts 2!" and then suddenly this will be the source of confusion as it may return one or the other post only, which will result in an eventual deprecation and breaking change to remove it in the future.

I just don't see this as something the library should implement.

[SSardorf]

Pinned posts are decided with a channel flag—it would be very easy for Discord to say "Hey, let's make the maximum pinned posts 2!" and then suddenly this will be the source of confusion as it may return one or the other post only, which will result in an eventual deprecation and breaking change to remove it in the future.

I just don't see this as something the library should implement.

Or we change it to filter on the Pinned channel flag instead of find in the event of an eventual change in the maximum amount of pinned posts

[Jiralite]

In the event of an eventual change in the maximum amount of pinned posts, modifying find() to filter() is a breaking change. The return type changing to a possible thread to a collection is not backwards-compatible.

If this is to happen, it needs to be a filter now. That being said, a filter returning only one result is strange.

I am thinking long-term here, and it seems something better left to consumers on how they want to get a pinned post.