RFC: Location for model builders

[7596ff]

Right now, we have a few different model builders in different places:

• crate embed-builder
  • EmbedBuilder
• crate model
  • AllowedMentionsBuilder
  • GuildChannelFieldsBuilder
  • RequestGuildMembersBuilder

There is (as of 2021-07-20) an open PR to add a Command builder to model. Another aspect of this problem is that our builders perform validation tasks for HTTP requests.

We need to decide what to do here to make things more consistent. From what I can see there are a few options.

• Move all builders to the util crate, with their own separate features
• Move the Embed builder to the model crate, which is consistent with the other builders
• Unify builders and validation in their own crate
  (This relates to Unify validation errors into more concise error types #999 because at the moment we need to update two different const values to update the embed validation)