feat(): replaced the List<String> for GuildFields features by an Enum…

[gmisiurny]

Description : Discord4J/Discord4J#1196 (comment)

Justification : Discord4J GuildFields interface rely on this repository.

[Doc94]

Why PermissionSet?

[gmisiurny]

I can change the name of the enum if you want. PermissionSet is what @Azn9 suggested.
Maybe GuildFeature or GuildPermission is more suitable ?

[Azn9]

Thank you for your contribution!

However, there are some changes required on this PR.
In fact, this PR is not needed. The conversion from a List to a EnumSet should be done only on the Discord4J part. The discord-json project aims to represent discord entities as immutables objects in their most basic form.
You can take example on the message data flags field for instance

discord-json/src/main/java/discord4j/discordjson/json/MessageData.java

Line 79 in cc21386

Possible<Integer> flags();

https://github.com/Discord4J/Discord4J/blob/026c1b8aa424d18679331102043ceab3d8e6fc74/core/src/main/java/discord4j/core/object/entity/Message.java#L427C1-L431C6

For the "PermissionSet", I just said "something like", to replace a EnumSet. Have a look at the file https://github.com/Discord4J/Discord4J/blob/master/rest/src/main/java/discord4j/rest/util/PermissionSet.java

[Doc94]

Closed in favor of Discord4J/Discord4J#1207
Like Azn9 this project represent the data in the most direct way.