[Feature] Support for the new username system

[Misha-133]

This PR brings support for the new username system introduced by discord.

Changes

• update all ToString() methods of user entities & their debugger display properties to use the new unique username instead of username#discriminator pattern if discriminator value is equals to 0 - the placeholder value.
• made discriminator parameter of BaseSocketClient.GetUser() method optional
• update EmbedBuilderExtensions.WithAuthor()
• update MentionUtils
• Add GlobalName property to user entities
• Add a new CDN method with support for new default avatar calculation
• Update UserTypeReader in Discord.Commands to search for GlobalName

Misc

• update MentionUtils to use <@id> user mention syntax instead of deprecated <@!id> (docs)

• closes [Bug]: New usernames are displayed with #0000 discriminator #2693

P.S. I think I've found all places that need to be updated for the new system, pls correct me if I missed any

[Panthr75]

Might want to deprecate Discriminator and DiscriminatorValue, as most users have already transferred over to the new username system

[d4n3436]

Might want to deprecate Discriminator and DiscriminatorValue, as most users have already transferred over to the new username system

Bots will remain on the old username system
https://support-dev.discord.com/hc/en-us/articles/13667755828631#h_01H26BDRT8ASH1YVKG6HMGZ1J2

[Panthr75]

Update SocketThreadUser.DisplayName to match SocketGuildUser.DisplayName by including a nullability check with GlobalName before Username