Add support for user primary guilds

[mjezek-dev]

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Description

Adds support for user primary guilds.

[Chew]

This seems to have a mix of tab and 4 space indention, please make sure it's consistently 4 spaces

[aveCard1nal]

May I also suggest adding:

List<Member> Guild#getMembersByPrimaryGuild()
RestAction<List<Member>> Guild#retrieveMembersByPrimaryGuild()

This could be useful for cases where bots need to identify members who selected the guild as their primary. For example, to assign roles, highlight core members, or provide exclusive features.

[mjezek-dev]

You could use Guild#findMembers with filter checking for member's primary guild to retrieve this info. Discord API doesn't directly support searching for members based on their primary guild like that is being done with usernames. Looking for cached members can be done just by iterating over list of all cached members from Guild#getMembers and checking for required primary guild, so I don't think it's needed to add specific method for this.

[FigT]

Hey! Is there anything blocking this PR? This would be useful :}